Well well well, look what the shoggoth dragged in.

Mechanics

Knowledge of the Future of the Universe (10%)

This is a homebrew skill I gave one of my Masks of Nyarlathotep players after they decided to become friendly with a time-traveller. 10% chance of answering a plot-relevant question, and immediate recognition of key NPCs.

Dice rolls in Call of Cthulhu

Call of Cthulhu uses a roll-under percentile dice system. It also has a notion of task difficulty and situational modifiers, and a way for two characters to directly compete. This post runs through all these variations, with lots of graphs, and gives my rules for deciding what sort of roll to make when.