Flying
Revenant's power and toughness are each equal to the number of creature cards in your graveyard.
As long as Revenant is in your graveyard, its ability will count itself.
If Revenant is dealt lethal damage at the same time as another creature you control, both die at the same time. The other creature won't die in time to raise Revenant's toughness.
The ability that defines Revenant's power and toughness applies in all zones, not just the battlefield.