The cost to activate a creature's outlast ability includes the tap symbol (). Envoy of the Ancestors's outlast ability can't be activated unless it has been under your control continuously since the beginning of your turn.

A creature with a counter on it is considered modified no matter what kind of counter it is or which player put it on that creature.

An Aura controlled by another player does not cause a creature you control to be modified.

A creature that is equipped is considered modified no matter who controls the Equipment that's attached to it.