When the game calls for a decision (where Blight cascades to, which Invaders to do Damage to, etc) and it’s not specified who makes it, what do you do?

If the decision springs from a Power, the Spirit using the Power makes the decision. For other situations, players should try to come to a consensus. But in the rare event that agreement is not possible:

For decisions about/within a land: Each Spirit makes choices regarding the lands on their starting board.

For other cases: The game owner/organizer makes the call.