Champion of the Fiend

Indomitable (2/Day). The champion rerolls a failed saving throw.

Dark One’s Own Luck (Recharges after a Short or Long Rest). When the warlock makes an ability check or saving throw, it can add a d10 to the roll. It can do this after the roll is made but before any of the roll’s effects occur.

Multiattack. The champion makes three Scimitar attacks.

Scimitar. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) slashing damage plus 14 (4d6) fire damage, plus 7 (2d6) slashing damage if the champion has more than half of its total hit points remaining.

Hellfire. Green flame explodes in a 10-foot-radius sphere centered on a point within 120 feet of the warlock. Each creature in that area must make a DC 15 Dexterity saving throw, taking 16 (3d10) fire damage and 11 (2d10) necrotic damage on a failed save, or half as much damage on a successful one. 

Second Wind (Recharges after a Short or Long Rest). The champion regains 20 hit points.

Hellish Rebuke (3/Day). In response to being damaged by a visible creature within 60 feet of it, the warlock forces that creature to make a DC 15 Constitution saving throw, taking 22 (4d10) fire damage on a failed save, or half as much damage on a successful one.