Arm of Vecna

Dark Devotion. The arm has advantage on saving throws against being exhausted or stunned.

Disciple of Vecna. The arm has advantage on Charisma (Deception) made to lie and Wisdom (Insight) checks to determine whether someone is lying to it. Additionally, the arm is immune to the detect thoughts spell and other magical effects that would read its mind.

Survivor. The arm regains 10 hit points at the start of its turn if it has fewer than half its hit points but at least 1 hit point.

Undead Commander. Any allied Undead within 15 feet of the arm adds a +3 to attack and damage rolls.

Multiattack. The arm uses Dreadful Aspect (if available) and makes three attacks using Battleaxe, Light Crossbow, and Maul in any combination.

Battleaxe. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 9 (1d8 + 5) slashing damage, or 10 (1d10 + 5) slashing damage when used with two hands, plus 14 (4d6) necrotic damage.

Dagger. Melee or Ranged Weapon Attack: +6 to hit, reach 5 ft. or range 20/60, one target. Hit: 4 (1d4 + 2) piercing damage plus 2 (1d4) necrotic damage.

Light Crossbow. Ranged Weapon Attack: +6 to hit, range 80/320, one target. HIt: 6 (1d8 + 2) piercing damage.

Maul. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage plus 9 (2d8) poison damage.

Necrotic Burst. Melee Spell Attack: +6 to hit, reach 5 ft., one creature. Hit: 9 (2d6 + 2) necrotic damage.

Dreadful Aspect (Recharges after a Short or Long Rest). Each enemy within 30 feet of the arm must succeed on a DC 15 Wisdom saving throw or be frightened of the arm for 1 minute. If a frightened target ends its turn more than 30 feet away from the arm, the target can repeat the saving throw, ending the effect on itself on a success. 

Spellcasting. The arm casts one of the following spells, using Charisma as the spellcasting ability (spell save DC 15):

1/day each: aid, aura of vitality, banishment, command, protection from evil and good, zone of truth

Command Ally. The arm targets one ally it can see within 30 feet of it. If the target can see or hear the arm, the target can make one weapon attack as a reaction and gains advantage on the attack roll.

Smite. Immediately after the arm hits a target with a melee attack roll, the arm can force that target to make a DC 15 Constitution saving throw. On a failed save, the target suffers one of the following effects of the arm's choice:

Blind. The target is blinded for 1 minute. The blinded target can repeat the save at the end of each of its turns, ending the effect on itself on a success.

Brand. The target is marked until the end of the arm's next turn. In that time, the arm or one creature it chooses has advantage on attack rolls against the target.

Shove. The target is pushed up to 10 feet away and knocked prone.