Multiattack

The nihileth makes three tentacle attacks or three withering touches, depending on what form it is in.

Tentacle (Material Form Only)

Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage. If the target creature is hit, it must make a successful DC 14 Constitution saving throw or become diseased. The disease has no effect for 1 minute; during that time, it can be removed by lesser restoration or comparable magic. After 1 minute, the diseased creature's skin becomes translucent and slimy. The creature cannot regain hit points unless it is entirely underwater, and the disease can only be removed by heal or comparable magic. Unless the creature is fully submerged or frequently doused with water, it takes 6 (1d12) acid damage every 10 minutes. If a creature dies while diseased, it rises in 1d6 rounds as a nihilethic zombie. This zombie is permanently dominated by the nihileth.

Withering Touch (Ethereal Form Only)

Melee Weapon Attack: +8 to hit, reach 10 ft., one creature. Hit: 14 (3d6 + 4) necrotic damage.

Form Swap

As a bonus action, the nihileth can alter between its material and ethereal forms at will.

Tail (Material Form Only)

Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 15 (3d6 + 5) bludgeoning damage.

Enslave (3/day)

The nihileth targets one creature it can see within 30 ft. of it. The target must succeed on a DC 14 Wisdom saving throw or be magically charmed by the nihileth until the nihileth dies or until it is on a different plane of existence from the target. The charmed target is under the nihileth's control and can't take reactions, and the nihileth and the target can communicate telepathically with each other over any distance. Whenever the charmed target takes damage, the target can repeat the saving throw. On a success, the effect ends. No more than once every 24 hours, the target can also repeat the saving throw when it is at least 1 mile away from the nihileth.