Arm of Orcus

Hybrid dead: These creatures are immune to non-magical damage and most magical damage, including necrotic, fire, radiant, acid, and physical.

Spellcasting: The Arm of Orcus is a 10th level spellcaster. Its spellcasting ability is 18 (spell save DC 16, +8 to hit with spell attacks). The Arm of Orcus has the following Sorcerer spells prepared: Cantrips (at will):Acid Splash,Chill Touch, Minor Illusion, Poison Spray, Ray of frost, and Shocking Grasp.
1st level (4 slots):Burning Hands, Disguise Self, False Life, Ray of Sickness, and Witch Bolt
2nd level(3 slots) Blur, Darkness, Enlarge/Reduce, and Web.
3rd level (3 slots) Dispel Magic, Fear, Stinking Cloud, and Water Walk.
4th level (3 slots)Blight, Dominate Beast, Greater Invisibility, and Wall of Fire.
5th level (2 slots)Animate Objects, Cone of Cold, Dominate Person, and Insect Plague.

Actions: Multi-Swipe. Action Melee Attack. Melee Weapon Attack: +8 to hit, reach 10 ft.,up to 4 targets independently. 28(5d8 + 5) bludgeoning damage.

Deadblood spit. Attack: +8 to hit, range 20/50 ft., 1 target. Hit: 21(3d10 + 4) acid damage.

Reactions: Deadblood Spray.  when hit by a slashing or piercing attack the creature sprays an area of up to 15ft with a noxious liquid that causes instant vomiting for 1d4 rounds preventing all actions but fleeing.

Legendary Actions: Body Roll:  When at 25% hit points the Arm of Orcus will roll toward the attackers, if they hit the targets - Attack: +8 to hit for the length of the current body - they take 15 (3d8+5) bludgeoning damage and are knocked prone

Assimilate Bodies: If they are near dead bodies they can absorb them in to it's form adding to its body. A normal medium humanoid will add 2-5 ft. to the body. When first encountered they are usually 10-30 ft. in length/height.