Amalgamental

Pool of Partition.  The Void Elemental cannot move from the Pool of Partition.

Radiant Instability. Whenever the Void Elemental is hit with radiant damage, it takes an additional 1d10 radiant damage and spawns one Voidling (see "Voidling" stat block) in an unoccupied space adjacent to it. The Voidling acts immediately after it in initiative order.

Void Zone. If a creature enters the space of the Void Elemental or attempts to grapple it, the creature takes 4d10 force damage and is thrown back 20ft.

Soul Collective. The Void Elemental's AC increases by 1 for each Voidling within 120ft.

Multiattack. The Void Elemental makes three attacks: one with it's Dark Beam, one with it's Glob Lob, and one with it's Pseudopod. After it has lost 100 HP, it can use Dark Chain or Plane Shift in place of one of these attacks. If a creature is already grappled by Dark Chain, it can use Draw Near as a free action once each turn.

Dark Beam. The Void Elemental fires a beam at an unobstructed target it can see within 120ft. The targeted creature must succeed on a DC 15 Dexterity Saving Throw or take 4d10 necrotic damage. Creatures within 10ft of the Void Elemental can roll with advantage. On a hit, 1d2 Voidlings spawn in unoccupied adjacent spaces next to the target and act immediately after the Void Elemental in initiative order.

Glob Lob. Ranged Weapon Attack: +7 to hit, reach 120ft, one target. Hit: 1d10 force damage. The affected creature is infused with shadow energy. At the start of their turn it bursts, dealing 1d10 necrotic damage. It also leaves a pool of inky blackness on the ground in that spot and all squares adjacent (3x3). Any creature that starts its turn in the pool or moves into it for the first time on their turn takes 1d10 necrotic. The pool lasts until the players Plane Shift.

Psuedopod. Melee Weapon Attack: +7 to hit, reach 10ft, one target. Hit: 1d6+2 force damage. The target must succeed on a DC 13 Constitution Saving Throw or it's hit point maximum is reduced by half the damage taken. This effect lasts until the affected creature takes a long rest, or if the Greater Restoration spell is used. The target dies outright if it's hp max is reduced to 0, and cannot be revived with anything but a Wish spell.

Dark Chain. The Void Elemental attempts to tether itself to a creature within 60ft that is at least 10ft away with a crackling shadowy chain. The target must succeed on a DC 15 Dexterity Saving Throw or be grappled (escape DC 18). A grappled creature takes 3d6+4 force damage at the beginning of each of it's turns, and half of any health it recovers is instead siphoned to the Void Elemental. If the Void Elemental uses Plane Shift, the grappled creature is taken with it. Any radiant damage inflicted on the chain immediately destroys it. Only one creature can be chained at a time.

Plane Shift. (Recharge 5-6). The Void Elemental teleports to the Feywild, Shadowfell, or Material version of it's lair. If it is grappling a creature with Dark Chain, the creature is pulled with it.

Draw Near. As a free action, the Void Elemental pulls a creature grappled by Dark Chain 5, 10, or 15 feet towards it (roll a D6, 1-2=5ft, 3-4=10ft, 5-6=15ft). If this brings the creature into the Void Elemental's space, they take 4d10 force damage and are thrown 20ft backwards, though the chain still remains.

The Void Elemental can take 1 Legendary Action per round, at the end of another creature's turn.

Create Voidlings. The Void Elemental spawn 2 Voidlings in unoccupied spaces within 120ft of itself. The Voidlings act after the Void Elemental in the initiative order.

Set Fuse. All Voidlings within 120ft of the elemental turn unstable and begin to swell. At the beginning of the Void Elemental's next turn, all the Voidlings detonate. Each creature within 10ft of a detonating Voidling must make a DC 15 Dexterity Saving Throw. A target takes 6d6 force damage on a fail, or half as much on a save. 

Vacuum. All Voidlings within 120ft of the Void Elemental are pulled in a straight line towards it. Any creature caught in the path of a vacuumed Voidling must succeed on a DC15 Dexterity Saving Throw or take 1d6+3 force damage. For every Voidling the Void Elemental absorbs, it regains 5 hit points.

Legendary Resistance 3/day.