Annihilating Fire deals 3 damage to any target. If a creature dealt damage this way would die this turn, exile it instead.
A creature dealt damage by Annihilating Fire that dies this turn will be exiled even if it wasn’t the target of Annihilating Fire (say, because the damage was redirected somehow).
A creature dealt damage by Annihilating Fire will be exiled no matter why the creature would die. It could be destroyed by another spell or ability.
The creature can still regenerate.