Whenever you cast a spell from your graveyard, Burning Vengeance deals 2 damage to any target.
Although flashback and retrace are the most common ways to cast a spell from your graveyard, Burning Vengeance will trigger whenever you cast a spell from your graveyard through any means.
Burning Vengeance doesn’t trigger when you activate an ability of a card in your graveyard, such as unearth or the ability of Reassembling Skeleton.
Burning Vengeance’s triggered ability will resolve before the spell you cast from your graveyard.