"Faith can quicken the stones themselves with life."
—Writings of Mikaeus

Angel's Tomb

Whenever a creature you control enters, you may have this artifact become a 3/3 white Angel artifact creature with flying until end of turn.

Innistrad Remastered

Magic Origins

Whenever a creature enters the battlefield under your control, you may have Angel's Tomb become a 3/3 white Angel artifact creature with flying until end of turn.

Avacyn Restored

Effects setting Angel’s Tomb’s base power and toughness to specific values that begin to apply after Angel’s Tomb has become a creature will override the effect of Angel’s Tomb. For example, if an effect causes a 3/3 Angel’s Tomb to become 0/1, it will remain 0/1 until another effect (such as triggering the ability of Angel’s Tomb a second time or targeting it with Titanic Growth) causes those values to change.

If Angel’s Tomb is already a creature when a creature enters the battlefield under your control, its ability will override any effects that set its base power and toughness to specific values, but other changes to its power and toughness (such as the one created by Titanic Growth) will still apply.