A roll of 100 (or 96+ if the target number is below 50).
When confronted by horror, roll against your current Sanity (SAN) . Success: Minimal or no SAN loss. call of cthulhu 7th edition cheat sheet
Triggered by losing 5+ SAN in a single roll if you also pass an Intelligence (INT) check (the "Idea" roll). A roll of 100 (or 96+ if the target number is below 50)
While the core d100 system handles most checks, the 7th Edition introduced "Advantage/Disadvantage" mechanics similar to modern D&D, but with a distinct flavor. but with a distinct flavor.