Aura of Dread.  A 30-foot-radius aura of necrotic energy surrounds the Behemoth. Creatures that start their turn within the aura must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A frightened creature can repeat the saving throw at the end of its turn, ending the effect on a success.

Grasping Tendrils. The Behemoth’s body is surrounded by shadowy tendrils that lash out at enemies. Creatures that end their turn within 10 feet of the Behemoth must make a DC 17 Dexterity saving throw or take 10 (3d6) necrotic damage and be grappled (escape DC 18).

Unstable Core. When the Behemoth is reduced to 0 hit points, it collapses into a singularity. All creatures within 30 feet must make a DC 18 Dexterity saving throw, taking 45 (10d10) necrotic damage on a failed save, or half as much on a successful one.

Magic Resistance. The Behemoth has advantage on saving throws against spells and other magical effects.

Magic Weapons. The Behemoth's attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Multiattack. The Void Behemoth makes three attacks. One with its Void Slam and two with its Grasping Tendrils. 

Void Slam. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 24 (3d10 + 7) bludgeoning damage plus 14 (4d6) necrotic damage. 

Grasping Tendril. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 19 (3d8 + 7) bludgeoning damage and the target is grappled (escape DC 18). 

Void Beam (Recharge 5–6).
The Behemoth fires a devastating ray of void energy in a 60-foot line that is 5 feet wide. Each creature in the line must make a DC 18 Dexterity saving throw, taking 45 (10d8) necrotic damage on a failed save, or half as much on a successful one.

Voidquake (Recharge 6).
The Behemoth slams the ground, creating a shockwave of void energy in a 20-foot radius. Each creature in the area must make a DC 18 Strength saving throw or take 36 (8d8) bludgeoning damage and fall prone. The ground in the area becomes difficult terrain until the end of the Behemoth’s next turn.

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

Void Slam: The Behemoth makes a Void Slam attack.

Grasping Flurry (Costs 2 Actions): The Behemoth makes two Grasping Tendrils attacks.

Necrotic Pulse (Costs 3 Actions): The Behemoth releases a surge of necrotic energy. All creatures within 30 feet must make a DC 18 Constitution saving throw, taking 21 (6d6) necrotic damage on a failed save, or half as much on a successful one.