{Eidolon of Great Revel}
{collateral damage}
Scenario:
End of opponents turn, I have an eidolon on the board, I sac him as apart of the casting cost for {collateral damage}.
Do I receive any damage from eidolon?
I do not believe I do, as the requirement for the cast is a creature sacrifice. Therefore the eidolon was in the graveyard before the spell was casted.
It's all about the order of casting a spell. Don't remember which comes first, putting it on the stack or paying costs. Checking now.
601.2h Once the steps described in 601.2a–g are completed, the spell becomes cast. Any abilities that trigger when a spell is cast or put onto the stack trigger at this time. If the spell's controller had priority before casting it, he or she gets priority.
That was many many less rulings than I thought I'd have to dig through. (I had copied and pasted all of rule 601 before noticing this....)
The Eidolon is not present when its ability would trigger so you will not take damage.