Sister Mary - Ability

By Roger the Alien, in Elder Sign: Omens

She is immune from the nightly mythos effect.

I didn't see in any of my games that she prevented some effects, could you please give me some examples of cases where her ability is triggered.

Thank you

Yeah, it isn't obvious at the start, but Mary's ability prevents mythos effects where you lose stamina/sanity or items. Essentially, it only triggers if the mythos effect affects an individual investigator somehow. It does not prevent the addition of doom tokens, monsters appearing, that sort of thing. That particular characteristic isn't always useful, since some AO's (like Yig, Azathoth, and Tsathoggua) pretty much only add doom and monsters. It will trigger occasionally against Hastur (who eats at your sanity a lot) and the harder AO's from the expansions. However, the use of locked dice can come in very handy, especially when there are a lot of them and the rest of your investigators are crippled. Tsathoggua has a unique monster that locks green dice, Hastur locks yellows a lot (which can trigger extra doom), and all the AO's make at least some use of green/yellow/red-locking adventures (and, to a lesser extent, monsters).

Thank you for the answer, does the effect also prevent mythos effects of monsters/locations at midnight?

I played vs. Hastur and there was an effect which let the whole team lose 1 stamina, I assumed Mary wouldn't be affected but she also always lost 1 stamina...

I've played Hastur many times and I don't think I've ever seen a mythos effect where you lose stamina playing him. I suspect what got you was a midnight effect from an adventure, which is different. Mary won't help you with midnight effects from monsters or adventures- she specifically counters mythos effects that affect investigators at midnight, not midnight effects themselves. Prioritize getting rid of those midnight effect adventures and monsters!