Game Mechanics of Call of Cthulu in Comparison to Dungeons and Dragons
First, it's not a d20 system. Instead it's a d100-based system: characteristics and skills have a score ranging from 0 to 100. When you perform a check, you roll and hope for below your score. For example, if you have an intimidate score of 65, you'd need to roll 65 or below to succeed. This helps with intuitive understanding of the changes: a score of 65 means you have a 65% chance to succeed. Contrast that with D&D where a score of 14 means you have a 70% change to succeed!
The GM doesn't need to come up with DC checks – those are built into the scores, with a little nuance:
- roll a 1 and it's a critical success
- roll less than 1/5 of your score and it's an extreme success
- roll less than 1/2 of your score and it's a "hard success"
- roll less than or equal to your score and it's a regular success
- roll more than your score and it's a fail
- if your score is below 50 and you roll 96 or above, it's a
critical failfumble - if your score is above 50 and you roll 100, it's a
critical failfumble
Rolls can be "pushed" once after they fail. Narratively, this represents failing and trying again with more focus, gusto, etc. As for gameplay, this introduces a "press-your-luck" mechanic: if you succeed the pushed roll, great. If you fail, the GM comes up with something extra bad for the failure.
Characters don't "level up" in CoC. Instead, when a character succeeds a skill check, they mark it on their sheet (there's a checkbox next to each for this). When a milestone is hit according to the GM (often at the end of a session), players make an Improvement Check for each successfully checked skill. For these, they roll a d100; if the roll is above their current score in that skill (or 95), they roll 1d10 and add that to their skill score. After all is done, all checkmarks are erased, ready for the next session.
What's neat is that succeeding at a skill generally means you get better at it, though the flip side is that the better you are at a skill, the more difficult it is to improve.
One of the resources available to players is Luck (between 0 and 100 points). After a skill check is rolled, Luck points can be spent to achieve success (or a greater level of success). A few caveats:
- a player can't spend Luck on a fumbled roll
- a player can't spend Luck on a few specific rolls (actually Luck checks, damage rolls, sanity rolls)
- a player can't spend Luck after they've pushed the roll
- a player doesn't get to check the box next to the skill if they spent Luck
While Luck doesn't get a checkmark next to it when a Luck roll is made, it receives an Improvement Check each time whether or not any points were expended. Luck cannot exceed 99.
Because of the focus on the Cthulu Mythos in the general world of Call of Cthulu, Sanity plays a large role. While that alone isn't necessarily a useful thing to add onto other systems, some of the mechanics are slick.
One of the skills that a player has is Cthulu Mythos, representing their knowledge of otherworldly and supernatural events, magic, and similar things. Players begin with a score of 0 for this stat and it doesn't benefit from Improvement Checks. Instead, it improves slightly (by one or a few points) after players experience such bizarre occurrences. While this makes the character better at skill checks in this field, it reduces their maximum Sanity by the same amount. So a Cthulu Mythos score of 10 means maximum Sanity is 90.
Sanity is a resource (between 0 and 100 points) somewhat like mental Hit Points. As characters experience unsightly things, magic, etc, they lose Sanity. They'll gain it back slowly over time or a little more quickly by seeking therapy. I also appreciate that there's both a physical and a mental aspect of health (hit points and sanity).