Archfiend's Vessel

Fiendish Resistance. The vessel has advantage on saving throws against being charmed, poisoned, or frightened.

Shortsword. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage and the target must make a DC 16 Constitution saving throw, taking 7 (2d6) poison or necrotic damage (vessel's choice) on a failure, or half as much on a success. Either the vessel or one of its allies then regains hit points equal to half the amount of poison or necrotic damage dealt.

Knife. Melee Weapon Attack: +4 to hit, reach 5 ft. or range 10/30 ft., one target. Hit: 2 piercing or slashing damage (vessel's choice).

Spellcasting. The vessel casts one of the following spells, using Charisma as the spellcasting ability (spell save DC 13, +5 to hit with spell attacks):

At will: eldritch blast, corruption, friends, cause fear, minor illusion, message

1/day each: crown of madness, enthrall, hellish rebuke, summon undead, banishment

Harness Fiendish Power (1/Long Rest). The vessel channels the terrible power of its patron archfiend and regains one "1/day each" spell slot.

Summon Fiendform (1/Day). In response to being reduced to 0 hit points, the vessel uses a last ditch effort to automatically kill itself, causing its body to disintegrate into dark energy. Their is then a 80% chance that the vessel then summons a horrific apparition of its fiendish master, which has the stat block of any Fiend of CR 5 or lower. The summoned fiendish apparition remains for 10 minutes or until reduced to 0 hit points.