Innocent Traveler // Malicious Invader

Flying
This creature gets +2/+0 as long as an opponent controls a Human.

At the beginning of your upkeep, any opponent may sacrifice a creature of their choice. If no one does, transform this creature.

Innistrad: Double Feature

Innistrad: Crimson Vow

I looked at her throat just now as she lay asleep, and the tiny wounds seem not to have healed.

When Innocent Traveler's ability resolves, the next opponent in turn order chooses whether they want to sacrifice a creature at all and which creature to sacrifice if so, then each other opponent in turn order does the same, knowing the choices made before them. Then all the chosen creatures are sacrificed at the same time. If no creatures are sacrificed this way, Innocent Traveler is transformed.