Why Call of Cthulhu Works Alone
Call of Cthulhu suits solo play better than most games. Investigators work alone more than heroes do, the stories are about uncovering things, and the rules are a single percentile roll. Chaosium itself publishes solo gamebooks, such as Alone Against the Flames. This page goes the other way: instead of a written adventure, you make your own with an oracle. You ask the yes-or-no questions a Keeper would answer ("Is the study door locked?"), and the dice decide.
Nothing on this page is from the rulebooks. The oracle, the scene check and the tables are tools for playing alone; anything that changes a rule is labelled house rule.
What you need
- An investigator built with the normal rules (see Creating an Investigator), or one of the 56 Ready-to-Play Investigators.
- Two ten-sided dice (one for tens, one for units) plus a spare tens die, or the tool on this page and the Dice Roller.
- A mystery to start from: a letter, a disappearance, an inheritance. A published scenario you haven't read works too, if you only look at what your investigator would find.
- A journal, or the session log below.
The Oracle
Ask a question that can be answered yes or no. Decide how likely a "yes" is, then roll d100 the way the game already handles odds: with bonus and penalty dice.
- Very likely: 2 bonus dice (keep the lowest tens)
- Likely: 1 bonus die
- Toss-up: a plain d100
- Unlikely: 1 penalty die (keep the highest tens)
- Very unlikely: 2 penalty dice
Then read the result as if you were rolling against a skill of 50, using the same success levels as a check:
| Roll | Answer |
|---|---|
| 01–10 (an Extreme success against 50) | Yes, and… You get it, plus something more. |
| 11–25 (Hard) | Yes. |
| 26–50 (Regular) | Yes, but… It's true, with a cost or a complication. |
| 51–75 | No, but… Not that, but something useful. |
| 76–90 | No. |
| 91–100 | No, and… Not that, and things get worse. |
Twist: if the tens die you kept and the units die show the same number (11, 22, … 99, or 00 with 0 for 100), something unexpected also happens. It leans your way on a "yes" and against you on a "no". Roll a spark if you need an idea.
Exact odds
Worked out from every combination of the dice. The tool on this page uses the same calculation.
| Likelihood | Yes, and | Yes | Yes, but | No, but | No | No, and | Any yes | Twist |
|---|---|---|---|---|---|---|---|---|
| Very likely | 27.1% | 30.1% | 30.2% | 10.8% | 1.7% | 0.1% | 87.5% | 7.3% |
| Likely | 19.0% | 24.5% | 31.5% | 18.5% | 5.5% | 1.0% | 75.0% | 8.2% |
| Toss-up | 10.0% | 15.0% | 25.0% | 25.0% | 15.0% | 10.0% | 50.0% | 10.0% |
| Unlikely | 1.0% | 5.5% | 18.5% | 31.5% | 24.5% | 19.0% | 25.0% | 11.8% |
| Very unlikely | 0.1% | 1.7% | 10.8% | 30.2% | 30.1% | 27.1% | 12.5% | 12.7% |
The table is symmetrical: "unlikely" is exactly the mirror of "likely". One bonus die turns a coin flip into a yes three times in four, which is also a good feel for how much a bonus die helps an ordinary check. Twists get a little more common as the odds drop, because penalty dice favour high rolls, where 77, 88 and 99 live.
Ask only questions where both answers would be interesting. If you already know the answer, or a "no" would end the story, just decide.
Scenes, People and Sparks
Scene check
When you start a new scene, decide what you expect to happen, then roll a d10 (the units die on its own):
| d10 | The scene is… |
|---|---|
| 1 | Interrupted: something else happens instead. Roll a spark. |
| 2–3 | Altered: as expected, but one important detail is different. Ask the oracle what. |
| 4–10 | As expected. |
NPC attitude
When you meet someone whose attitude isn't obvious from the fiction, roll a d10:
| d10 | Attitude |
|---|---|
| 1 | Hostile: wants you gone, or worse. Possibly for reasons of their own. |
| 2–3 | Suspicious: guarded; social skills are a Hard roll. |
| 4–7 | Neutral: polite, busy, will help if it costs them nothing. |
| 8–9 | Helpful: answers questions and points the way. |
| 10 | Confiding: tells you something they've never told anyone. |
Sparks
Roll d100 and read the tens digit as an action and the units digit as a subject. Interpret freely; the first idea that fits is usually right.
| Digit | Action (tens) | Subject (units) |
|---|---|---|
| 0 | Conceal | a letter |
| 1 | Summon | a family secret |
| 2 | Follow | a cult |
| 3 | Warn | a book |
| 4 | Steal | a witness |
| 5 | Bargain for | a grave |
| 6 | Awaken | an artifact |
| 7 | Betray | a ritual |
| 8 | Destroy | a house |
| 9 | Reveal | the sea |
Playing the Keeper's Part
Checks, pushing and Luck
Your investigator's own actions use the normal rules. Set the difficulty honestly before you roll: Regular for most things, Hard when the task or an opponent is tough, Extreme when it's nearly impossible. Pushing works as usual. When a pushed roll fails, the consequence should hurt: ask the oracle "Is it as bad as I fear?" at Likely, and roll a spark for what goes wrong. Spending Luck (an optional rule) works well solo, because it lets you decide which failures matter.
The doom clock
House rule: the doom clock. A Keeper paces the horror; alone, a clock does it. Mark one point of doom each time the oracle answers No, and… or gives a twist on a "no", and each time you fail a pushed roll. When doom reaches 3, clear it: the next scene is interrupted without a scene check, by the thing you have been investigating. It has noticed you.
Sanity alone
Use Sanity exactly as written. Losses look smaller on paper than at a table, so write a line in your journal each time you lose Sanity: what your investigator saw and what they told themselves about it. If they go insane, use the summary version of a bout of madness (you come to some hours later, somewhere else) and ask the oracle what you did.
A party of one
- A companion (house rule): give your investigator one ally, such as a colleague, a sibling or a reporter friend, run as a simple NPC with three or four skills. They can make a roll you don't have the skill for, once per scene.
- Make fights rare: a lone investigator rarely survives one. Treat combat as a sign that something has gone wrong, and look for another way out.
Solo Tool
Doom: 0 of 3
Session log
The log is kept in this browser only.
Your First Solo Session
- Make an investigator and a hook. One person and one reason to start digging: a dead uncle's papers, a patient's drawings, a newspaper notice.
- Frame the first scene. Write one line: where you are and what you expect. Make a scene check.
- Play it. When you'd ask a Keeper something, ask the oracle. When your investigator tries something with a real chance of failure, make a normal check, set the difficulty first and push or spend Luck if you must.
- Follow the clues. Each scene should end with something new: a name, an address, a book. If it doesn't, ask the oracle "Is there something here I missed?" at Likely, the way an Idea roll would.
- Watch the doom clock. When it fills, the mystery comes looking for you.
- Stop after three or four scenes. Write down what your investigator knows, fears and still has to do. That's next session's opening.
An Example of Play
Harriet Lowe is a librarian in 1920s Arkham: Library Use 70, Spot Hidden 45, Sanity 55, Luck 50. A professor who borrowed a rare book from her library has vanished, and the book with him.
Scene 1: the professor's office. I expect a locked, dusty room. Scene check: d10 shows 6, so it's as expected.
Oracle: "Is his journal still here?" Likely, so one bonus die. The tens dice show 60 and 20, the units die 3. I keep the better tens: 23, which is 11–25: Yes. The kept tens (2) and the units (3) don't match, so no twist.
A real check. The journal is in a private shorthand. Harriet tries to make sense of it with Library Use, a Regular roll: 48, under 70 but not under 35 (half), so a Regular success. The last entry names a farmhouse outside town and copies a passage from the missing book.
Sanity. Reading the passage costs 0/1D3. She rolls 62 against Sanity 55: a failure, so she loses 1D3, which comes up 2. Sanity 53. Not 5 or more, so no INT roll.
Pushing. She searches the desk for the farmhouse's exact location: Spot Hidden 45, roll 71, a failure. She could spend 26 Luck (71 − 45) to succeed, but she pushes instead: she takes the drawers out one by one and checks underneath. The pushed roll is 33, a success (under 45, over 22). Taped under a drawer is a hand-drawn map.
Scene 2: the farmhouse road at dusk. I expect an empty road. Scene check: d10 shows 1, interrupted. Spark: d100 shows 47, tens 4 and units 7: steal a ritual. Someone has got here first: there are fresh tyre tracks and a dropped page from the missing book.
Oracle: "Is anyone still at the farmhouse?" Toss-up: a plain roll of 77, which is 76–90: No. But the tens and units match (7 and 7), so there's a twist on a "no": it goes against her. The farmhouse is empty, and the cellar door has been nailed shut from the outside. Doom: 1.
That's the loop: ask, roll, interpret, write a line, move on. Whether Harriet opens that cellar is a question no oracle can answer for you.
Where to Go Next
- Dice Roller: Your investigator's real checks, with exact odds, Luck and Sanity rolls.
- Investigation: How clues are laid out, which helps you ask the oracle better questions.
- Keeper Information: Pacing and dread, which matter more when you are your own Keeper.