Immunity
This object can't get damaged, can't gain condition and can't be affected by effects of the specified type.
So, this way we would be able to
1- play a Circle of Fire on a Flaming Hellion
2- play Plagued on a Necromancer
3- play a FD Ghoul Rot on a Necromancer ( to check for Nullify ) - turning it face up would be useless and a mana waste
4- Attack a guarding creature with immunity to that type of attack
If all "yes" , I'm all for it
This definition won't allow for 1 to happen (or rather it will happen, but FH won't benefit from CoF).
I wrote it that way because I thought "effect" was a reference to the Effect Die. But now browsing the rules, I notice it is use wildly for very different thing (Push effect, Teleport effect, Bloodthirsty seem also to be an effect according to Taunt).
So first, "effect" need to be clarified. Could you ask Brian, Laddinfance?