Not affecting Tarmogoyf means that this card begins the game outside of your deck, hence it's some unusual card type. The four cards that are related help me narrow it down in the following order:
Vanguard's Shield -- the card hinted at is a vanguard.
Providence -- the card we're looking for changes your life total to 26! This brings our options to Barrin, Karn, Rith, and maybe Stuffy Doll's vanguard versions if we allow for changes of -6.
Barrin and Karn are both cards, so we can restrict our investigation to those two. Karen's ability doesn't have anything in common with the listed cards, but Barrin's requires you to sacrifice a creature, just like Fanatical Devotion. Therefore we can conclude that the card we're looking for is Barrin's Vanguard card. All in all, a good riddle! (Assuming I got it right. If I didn't, clearly it's a very poor riddle /s)