Bloodfiend

Blood Possession. The Bloodfiend is physically indistinguishable from its host and cannot be differentiated by non-magical means, though it can cause its eyes to glow red if it chooses.

If it is reduced to 0 hit points and has been inside its host for less than 24 hours, the host regurgitates it, then falls unconscious with 1 hit point.

When possessing a host, any damage taken applies to the Bloodfiend and the host.

The possession does not end if the host drops to 0 hit points. Further attacks will accrue Death Saving Throw losses. When the host dies, the Bloodfiend is ejected from the host.

Devil's Sight. Magical darkness doesn't impede the devil's darkvision.

Flame Invigoration. Whenever the Bloodfiend takes fire damage it gains 2d10 temporary hit points and recharges its Flame Breath.

Cold & Radiant Aversion. When the Bloodfiend takes Cold or Radiant damage, it must make a concentration (constitution saving throw) check against a DC 10 or half of the Cold or Radiant damage taken (whichever is higher) or be ejected from the current host.

Multiattack. The Bloodfiend makes two melee attacks and uses its Flame Breath, if available. It can replace of of its Claw attacks with Invade if the target is grappled.

Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target.Hit: 6 (2d6 + 3) [piercing] damage and the targets must succeed on a DC15 strength saving throw or become grappled (DC15 to escape). A creature that is hit with a claw attack while surprised is automatically grappled. It can only grapple one creature at a time.

Invade. The Bloodfiend forces itself down the throat of a creature it is grappling. The target must make a Strength (athletics) or Dexterity (acrobatics) check, contested by the Bloodfiend. If the Bloodfiend succeeds, it possesses the body of the target, gaining 2d10 temporary hit points and control over the targets body. Its other stats, abilities, and hit points remain the same, and any active effects on the target continue as it is possessed.

Flame Breath. A line of roaring flame 30 feet long and 5 feet wide emanates from the host body in a direction chosen by the creature. Each creature in the line of fire must make a DC15 Dexterity saving throw. A creature takes (4d8) [fire] damage on a failed save, or half as much damage on a successful one.

Hellish Rebuke. After taking damage from a creature within 60 feet you can see, you point your finger and the creature that damaged you is momentarily surrounded by hellish flames. The creature must make a Dexterity saving throw against the Bloodfiend's spell DC. It takes (2d10) [fire] damage.