The Thunderbook — A Guide to Using a Brontologicon as an Artefact in OSR Play
“The Etruscans do not believe that things have meaning because they happen, but that they happen for the sole purpose of meaning.”
— Seneca, Natural Questions II.32.2
A thunderbook (or brontologicon) is a medieval prognostication manual that converts the sound of thunder into a forecast. In an OSR game it becomes a readable magic item: a small, weather-stained codex whose entries let a PC divine the future — at a cost. This guide gives you the artefact, the procedure for consulting it, ready-to-roll lookup tables converted from the historical genre, new spells, and a mishap system for when the book’s attentions turn on its reader.
It is written system-light. Where a die or save is referenced, use your rulebook’s nearest equivalent (e.g., Save vs. Wands, Save vs. Spell, a Wisdom check, or a flat d6 roll-under). Everything is a ruling, not a rule.
1. The Artefact
The Brontologicon (the “Thunderbook”)
Form: A palm-sized vellum codex, 12–24 folios, written in a crabbed ecclesiastical or necromantic hand. Some copies are illustrated with zodiac-man and vein-man diagrams, month labours, and a rota of winds. The best copies smell faintly of burned air.
Rarity: Uncommon in treasure hoards, monastic libraries, and the baggage of wandering diviners. A genuine Pseudo-Bede De tonitruis is worth 1,000–3,000 gp to a scholar, astrologer, or cult; a cursed or annotated copy is worth half that and twice the trouble.
Charges & decay: A thunderbook does not hold charges in the wand-staff sense. Instead, each full reading consumes one omen from the book. A typical codex holds 3d6+6 omens before its pages begin to go blank — the ink literally fading from the vellum as the future is “spent.” A depleted book is just vellum (and a plot hook: where did the spent futures go?).
Who can read it: Anyone literate in its language may read an entry. To activate an omen — that is, to make the book answer — the reader must (a) be present at a thunder event and (b) make a successful divination check: Save vs. Wands, or a Wisdom/INT roll-under, referee’s choice. Magic-users, elves, and clerics read at +2 (or advantage); fighters and thieves read at the base chance. On a failed check the omen is still spent, but the page returns only gibberish or a personal warning.
Alignment & taboo: Most thunderbooks are Neutral. A few are Chaos-touched — their omens come true, but always twist toward ruin. Reading one during a storm you did not summon is safe; reading one during a storm you summoned (by spell, item, or pact) doubles the mishap chance.
2. Using the Book at the Table
The procedure
- Thunder occurs. Either the referee rules that a storm breaks (weather check, travel hazard, or scene-setting), or a PC produces thunder by spell or item (see Call Thunder, below).
- A PC opens the book and reads. This takes 1 round per table consulted and requires both hands free. The reader cannot fight or defend while reading.
- Choose a key — the axis along which the thunder is interpreted: the month, the zodiac sign, the weekday, the hour, or the direction the thunder came from. The reader picks one; the referee may allow a second key for +1 omen cost (a “cross-referenced” reading, more specific but dearer).
- Roll on the matching table (§3). The result is the omen: a short, gnomic forecast that the referee interprets as a truth about the near future of the party or the region.
- Pay the cost. Cross off one omen. On a natural 1 on the table die, also roll on the Mishap table (§5).
How omens play out
An omen is not a command or a buff. It is a disclosed fact about what will happen. The referee should treat the rolled omen as a true statement about the next 1–7 days of game time and shape events, encounter checks, and rumours to fulfil it. This is the genre’s whole engine: thunder happens so that it can be read.
| Omen says… | The referee should… |
|---|---|
| “abundant crops” | raise the region’s morale and provisions; lower food prices. |
| “bloodshed” | trigger a hostile encounter, a feud, or an ambush within the window. |
| “death of a great one” | a named NPC dies or falls; the party hears of it. |
| “plague” | begin a disease timer; encounters with the sick appear. |
| “a king’s downfall” | a political event reaches the party as rumour, refuge, or bounty. |
| “false reading / silence” | the omen is void; the omen is still spent. |
The players may act on an omen (leave town before the bloodshed, buy grain before the famine). That is the entire point of the item and should be rewarded with good play, not punished.
Encumbrance & risk
- A thunderbook is 1 slot (or 1 stone / 3 coins, per your system).
- Reading it in heavy rain or wind is fine — that is its element. Reading it underground or indoors imposes −4 on the divination check: the book wants the open sky.
- The book must be read aloud to work. Anything within earshot can hear it. This includes things that listen for names being spoken.
3. Lookup Tables (Converted for OSR Use)
Each table below is keyed to one historical axis of the brontologicon. Use the one that matches how the PC observed the thunder. Results are written as protasis–apodosis entries, in the book’s voice — read them aloud to the player.
Designer note — converting to your setting. These tables are generic. To re-skin them to your campaign: (a) replace the month names with your calendar’s months; (b) replace “king” with your nearest sovereign or faction head; (c) replace crop/livestock results with whatever your region’s economy runs on (spice, souls, ore, slaves). The structure is what makes it a brontologicon: a fixed key, a short forecast, a cost.
3.1 By the Month (roll 1d12)
Use when the thunder is heard during a known month. This is the most common key and the one most medieval copies lead with.
| 1d12 | If it thunders in… | …the omen is: |
|---|---|---|
| 1 | January | Death walks the high places. A named NPC will die before the snow melts. |
| 2 | February | Strong winds and much bloodshed. +2 to encounter rolls outdoors for 1d6 days. |
| 3 | March | The earth stirs ill. Crops fail in one region; food prices double. |
| 4 | April | Rain and renewal. Travel is slow (−25% movement) but no disease this month. |
| 5 | May | A great one falls from favour. A ruler or lord is deposed or publicly shamed. |
| 6 | June | Destruction of the crop, save the barley (or your setting’s hardy staple). Famine localised to one settlement. |
| 7 | July | Pestilence. Begin a disease timer: one PC or retainer is exposed. |
| 8 | August | War is kindled. A faction takes the field within the month. |
| 9 | September | A false omen — the book falls silent. The reading is void; the omen is still spent. |
| 10 | October | Discord among rulers; the common folk press. Morale checks for hirelings at −2 for 1 week. |
| 11 | November | Death of cattle (or the setting’s chief livestock/wealth-beast). Herd losses reported. |
| 12 | December | Health and prosperity. All party members heal +1 HP per night’s rest for 1 week. |
3.2 By the Sign of the Zodiac (roll 1d12)
Use when the sun is known to be in a given sign. Many brontologia key on the zodiacal month rather than the calendar month; this table is rolled by the referee using the campaign date, not chosen by the player.
| 1d12 | Sign | If it thunders while the sun is in… | …the omen is: |
|---|---|---|---|
| 1 | Aries | A leader is born or crowned. A new faction head appears within 1d6 days. | |
| 2 | Taurus | Wealth moves. A treasure hoard shifts location; the party hears a true rumour of it. | |
| 3 | Gemini | Twins’ omen — a betrayal by someone close. A retainer or contact turns. | |
| 4 | Cancer | Retreat and flood. Water rises; one route becomes impassable. | |
| 5 | Leo | The proud are struck. A powerful figure suffers public defeat or illness. | |
| 6 | Virgo | The harvest omen. Provisions are plentiful; forage yields double for 1 week. | |
| 7 | Libra | Justice or vengeance. A wrongdoing from the party’s past resurfaces. | |
| 8 | Scorpio | Poison and treachery. Save vs. Poison for one random PC; on a fail, 1d6 days of weakness. | |
| 9 | Sagittarius | Horsemen and strangers. Mounted encounter within 1d3 days. | |
| 10 | Capricorn | The mountain stirs. A quake, avalanche, or collapse affects one travelled route. | |
| 11 | Aquarius | Floods and portents. A river changes course; a map becomes wrong. | |
| 12 | Pisces | Drowning and depths. Any water travel this week carries a 2-in-6 hazard. |
3.3 By the Day of the Week (roll 1d8; on an 8, reroll)
The seven-day key, from the De tonitruis. The 8th face of the die is the book’s silence — reroll.
| 1d8 | Day | If it thunders on… | …the omen is: |
|---|---|---|---|
| 1 | Sunday | The divine turns its face. Cleric spells this day are cast at −1. | |
| 2 | Monday | Journeys go astray. Any travel this week has a 1-in-6 chance of wrong-turn or delay. | |
| 3 | Tuesday | Battle is favoured. +1 to the party’s side in the next armed encounter. | |
| 4 | Wednesday | Messages and lies. A letter or rumour reaches the party — half-true. | |
| 5 | Thursday | A ruler’s day. The party is summoned, hired, or summoned by a power. | |
| 6 | Friday | Love and ruin. A romance or alliance forms; another breaks. | |
| 7 | Saturday | The old powers stir. Undead activity +1 for 1d6 days. | |
| 8 | (silence) | The book will not say. Reroll. |
3.4 By the Hour (roll 1d12)
The canonical-hour key. The hour is the hour of the thunder, not the reading. Eleven historical hours are mapped to twelve faces; the 12th is the book’s drift.
| 1d12 | Hour | The omen: |
|---|---|---|
| 1 | First hour (dawn) | A beginning. A new hook, rumour, or door opens today. |
| 2 | Third hour | Labour’s sign. One crafting or building project completes early. |
| 3 | Sixth hour (midday) | The sun’s judgement. A secret is exposed before nightfall. |
| 4 | Ninth hour | The sun declines. A resource runs short (torches, rations, or a spell slot). |
| 5 | Tenth hour | The dead hour. A ghost or memory of the past appears. |
| 6 | Twelfth hour | The threshold. A door, gate, or passage opens that was sealed. |
| 7 | Dusk | Between days. A creature of dusk crosses the party’s path. |
| 8 | First vigil | The watcher’s sign. A wandering encounter occurs tonight. |
| 9 | Middle of the night | The deep omen. One PC has a true, ominous dream. |
| 10 | Twilight (before dawn) | The thinning. The boundary to another place wears; one encounter this day is otherworldly. |
| 11 | Dawn (again) | Renewal. One negative condition is lifted at sunrise. |
| 12 | (drift) | The hours blur. The omen is void; the omen is still spent. |
3.5 By the Direction (roll 1d6 or 1d8)
From the cardinal-direction chapter of the De tonitruis. Use 1d6 for the four winds plus two “ill” faces; use 1d8 for the eight winds (add the diagonals). The direction is the direction from which the thunder came, ruled by the referee.
1d6 (compact):
| 1d6 | From… | The omen: |
|---|---|---|
| 1 | North (cold wind) | Hardship and cold. Travel costs doubled; cold-weather checks begin. |
| 2 | East (dawn wind) | New things come. A stranger, rumour, or opportunity within 1d3 days. |
| 3 | South (warm wind) | Plague or plenty (50% each). |
| 4 | West (death wind) | A death in the party’s circle within the week — retainer, contact, or animal. |
| 5 | From everywhere at once | The book shudders. Roll on the Mishap table. |
| 6 | From nowhere (silence) | Void; omen spent. |
1d8 (eight winds, expanded): add NE (betrayal from within), NW (a lost thing returns), SE (a journey forced upon the party), SW (a debt comes due).
4. Spells
A genuine brontologicon is often also a spellbook: it contains the spells needed to make thunder when the sky will not. The spells below are written in the OSR idiom — terse, one-paragraph, save-or-suffer. Use your system’s nearest level equivalent. Unless noted, all are Magic-User spells; clerics of storm or sky cults may take those marked (C).
1st Level — Brontomancy (Divination)
Range: Self. Duration: Instant. Save: None.
By speaking an entry from a thunderbook aloud during a thunder event, the caster reads one true omen about the next 1d6 days. Roll on any one thunderbook table; the result is a true statement about the near future. The caster need not own the book, but must read from a genuine brontologicon. A given caster cannot benefit from brontomancy more than once per thunder event.
1st Level — Call Thunder (Evocation)
Range: 120 ft. Duration: Instant. Save: None.
The caster summons a single, sharp clap of thunder from a clear or clouded sky. It does no damage, but is loud enough to trigger a thunderbook reading, startle beasts (morale check), reveal position, or shatter fragile glass within 20 ft. The clap is audible out to 1 mile. The caster may choose the apparent direction from which it came.
Designer note: This is the workhorse spell that makes the thunderbook usable on demand. Its cost is the spell slot; the book’s cost is the omen.
2nd Level — Fulgurmancy (Divination) (C)
Range: Self. Duration: 1 turn. Save: None.
The caster may read lightning as well as thunder. During a thunderstorm, the caster may ask one yes/no question per bolt of lightning observed; the referee answers truthfully, interpreting the bolt’s path, colour, and timing. A false or evasive answer accrues a debt to the storm (see Mishaps).
3rd Level — Voice of the Brontologion (Enchantment/Compulsion)
Range: 30 ft. Duration: 1d6 days. Save: vs. Spell negates.
The caster reads an omen aloud and imposes it on a target who can hear: the omen becomes a geas-lite truth about that target’s near future. The target gets a Save vs. Spell to resist; on a fail, the referee schedules the omen’s fulfilment around the target within 1d6 days. A remove curse ends the effect. The omen is spent from the book whether or not the target saves.
4th Level — Stormcall (Evocation)
Range: Self (360 ft. radius). Duration: 1 turn per level. Save: None.
The caster summons a full thunderstorm: heavy rain, high wind, lightning. Movement in the storm is at half speed; missile attacks are at −4; small fires are extinguished; torches blow out on a 3-in-6. Lightning strikes strike once per round at a random target within the area (1d6 damage, Save vs. Wands for half). The storm provides unlimited thunder events for the duration — a thunderbook reader may consult the book once per round. Risk: summoning a storm you then read from doubles the mishap chance.
5th Level — Fulgur’s Bolt (Evocation)
Range: 360 ft. Duration: Instant. Save: vs. Spell for half.
The caster calls down a single aimed lightning strike. 8d6 damage to the target (Save vs. Spell for half); the bolt also deafens for 1d6 turns on a failed save. The strike leaves a scorched mark that radiates faint magic for 1d6 days. A target killed by Fulgur’s bolt cannot be raised by means lower than 7th level.
6th Level — The Brontologion’s Last Word (Divination)
Range: Self. Duration: Instant. Save: None.
The caster reads the final remaining omen in a thunderbook, then closes it forever. The spent omen is not a forecast but a command: the caster names one future event of the referee’s choosing within the referee’s judgment of scale, and that event will occur within the year. The book goes blank. The caster ages 1d10 years and must Save vs. Spell or lose 1 point of Wisdom permanently. This spell cannot be learned from a scroll; it can only be cast from a genuine, not-yet-depleted brontologicon.
5. Mishaps
OSR magic is dangerous, and a book that reads the future is more dangerous than most. Roll on the Mishap table whenever:
- the table die comes up natural 1, or
- a reading is attempted during a storm the reader or an ally summoned (double the chance — roll twice, take the worse), or
- a depleted book is forced to speak (see The Last Word, above).
Mishap (roll 1d8):
| 1d8 | What happens: |
|---|---|
| 1 | Spent. The omen is void and the book loses an extra omen. |
| 2 | Wrong ear. The omen is read aloud — but a listening creature (referee’s choice) now also knows it, and will act to profit from or prevent it. |
| 3 | The book bites. The reader takes 1d6 damage as a small bolt arcs from the page. |
| 4 | Reversed omen. The omen comes true inverted (plague becomes health, prosperity becomes famine). |
| 5 | Marked. The reader is now visible to storm-spirits; for 1d6 days, any storm within 6 miles will drift toward them. |
| 6 | Debt to the sky. The next time the reader casts a weather spell, it fails, and the slot is lost until they stand in a natural storm. |
| 7 | The book speaks unbidden. At the next inopportune moment, the book reads itself aloud, revealing one party secret to whoever is near. |
| 8 | The reader is read. The book answers not about the world, but about the reader: a true, unpleasant fact about the reader’s near future is stated aloud by the book. |
A mishap can never be “used up” — the risk persists as long as the book is used. This is intentional. The referee should let the costs accumulate until the player decides the book is more trouble than it is worth, and then let them sell it, bury it, or pass it on — which is, of course, when it becomes someone else’s problem.
6. Placing a Thunderbook in Your Campaign
As treasure: Put a thunderbook in a wizard’s library, a monastic computus collection, or the loot of a dead oracle. Value it like a scroll: useful, finite, a little cursed, definitely wanted by someone.
As a hook: A depleted book’s spent futures have gone somewhere. A scholar will pay well for a genuine one. A cult believes a particular thunderbook names the day of the world’s end. A king has outlawed them because of a reading he did not like.
As a recurring NPC: Give the book a voice — the marginal annotations of previous readers, who each came to a bad end. Let the book correct the reader. Let it refuse to be read indoors. Let it recommend itself to be stolen.
As a subsystem you can drop in: The whole guide is a self-contained minigame — storm, table, omen, cost. It needs no setting-specific lore and slots into any hexcrawl, city crawl, or dungeon-approach where weather and time of day are already tracked.
Designed for Old School Renaissance play. Adapt freely; the book expects to be wrong about something.