Aetherdrainer Squid

Amphibious. The Squid can breathe air, water, or in space.

Freedom of Movement. The Squid ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. It can spend 5 feet of movement to escape from nonmagical restraints or being grappled.

Siege Monster. The Squid deals double damage to objects and structures.

Magic Immunity. The Squid can make a Constitution saving throw against the spellcaster's spell save DC to resist any spell cast on it. If it succeeds, it instead regains 2d4 hit points per level of the spell, and 1d4 for cantrips. 

Spellcasting. The Squid is a 10th-level spellcaster. Its spellcasting ability is charisma. It has the following wizard spells prepared:

3rd level (6 slots): Counterspell, Dispel Magic

Hide of Shadow. No light escapes the squid's black skin. it has advantage on stealth checks while in dim light or darkness.

Multiattack. The Squid makes three tentacle attacks, each of which it can replace with one use of Infect.

Tentacle. Melee Weapon Attack: +17 to hit, reach 30 ft., one target. Hit: 20 (3d6 + 10) bludgeoning damage, and the target is grappled (escape DC 18). The Squid has 20 tentacles, each of which can grapple one huge or smaller target.

Infect. One target grappled by the Squid must succeed on a DC 20 Constitution saving throw or take 16 (1d12 +10) piercing damage and become infected by the squid. If the tentacle is destroyed, AC 24, 50 hit points, the effect ends for the creature.

While infested, the creature takes 16 (3d10) necrotic damage at the start of each of its turns, while the squid begins to eat them from the inside, and the squid regains that many hit points. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself after it succeeds on three of these saves.

If the target dies while infected by the squid, it puppets the now-hollow corpse. The squid can have as many puppets as it has tentacles.

The Squid can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Squid regains spent legendary actions at the start of its turn.

Tentacle Attack. The Squid makes one tentacle attack.

Cast a spell (Costs 2 Actions). The Squid casts a spell.

Basic Spray (Costs 3 Actions). Pores open across the skin of the Squid and a basic solution sprays out. All Creatures in a 40 ft. radius sphere centered on the Squid must make a DC 20 Dexterity saving throw, taking 18 (4d8) acid damage on a failed save, or half as much damage on a successful one. This damage ignores resistance or immunity to acid damage. 

Puppet (Costs 3 Actions). All Targets killed by the squid's Infect ability that are grappled are reanimated under the squid's control. The targets remain under the squid's control until they die or the tentacle that controls them is destroyed. The targets' stats are replaced by the stats of an Aetherdrainer Squid Puppet and they share the same initiative count as the squid taking their turns imidiatly after the squid.