I'm going to start with a realistic scenario, and move into the theoretical, so be prepared for that. (Full disclosure: this is more relevant to a homebrew character I made, but I'd like a more general ruling).
For Sigil of Wrath, it states that if you would fail a check with it, the monster is evaded (I think, I don't have it in front of me for exact wording). I was wondering how this would work with a monster that could not be evaded. I assume that in that case, he is just undefeated as normal, correct?
Following up from that, what would happen if there were a creature that could not be evaded, but had something like the ghost power where it is undefeated if you don't have the magic trait. I'm not sure if the powers interact at all (you would succeed at the check, but the monster would be undefeated), but I'm not sure.