This is an important interaction between two Journey into Nyx cards. Really it's a question about how "unless" interacts with targeting.
Say I have an Athreos, God of Passage (spoiler link) in play:
Whenever another creature you own dies, return it to your hand unless target opponent pays 3 life.
My opponent has an Aegis of the Gods (spoiler link):
You have hexproof.
What happens when one of my creatures dies, but there are no valid targets for the trigger?
- Do I get to return it to my hand from the graveyard because there is no other valid option?
- Or does it stay dead because the whole trigger is a no-go?