Hellfire Soul is a 15th level warlock Daily power for the Infernal Pact that was introduced in the 372nd edition of Dragon (magazine).
Hellfire Soul
Warlock (Infernal) Attack 15
Drawing upon your infernal ties, you set a foe's soul afire.
Ranged 10
Target: One creature
Attack: Constitution vs. Will
Hit: 3d10 + Constitution modifier necrotic and fire damage.
Miss: Half damage.
Effect: If this attack would reduce the target to 0 hit points or fewer, you can instead reduce the target to 1 hit point and the target is dominated by you until the end of the encounter.