Astral Form. The unseen one is invisible and can only be seen by true sight. The unseen one is also constantly under the effect of the nondetection spell.

Birth from Death. If the unseen one's hit points are reduced to 0 then it is immediately revived and cloned out of the nearest unseen one as if it has taken a long rest.

Hunger for Life. The unseen one does not need food, air, water, or sleep to survive instead it is required to kill one being every month in order to maintain its Astral Form and Birth from Death traits.

Immortality. The unseen one does not age and does not die from old age.

Legendary Resistance (3/Day). If the unseen one fails a saving throw, it can choose to succeed instead.

Spacial Armor. The unseen one adds its wisdom modifier to its AC.

Weapons of the Mirage. The unseen one conjures a magical weapon that cannot be removed from its grasp and counts a magical weapon to surpass resistance and immunity. This weapon can take any form and can be formed or dismissed as a bonus action by the unseen one. The weapon disintegrates when the unseen one dies.

Teleport. The unseen one can use a bonus action to teleport up to 300 feet to any unoccupied place it can see.

Multiattack. The unseen one can make 4 attacks with its Weapon of the Mirage.

Weapon of the Mirage. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 42 (5d12 + 10) of any type of damage they choose. 

Disintegration (Recharge 6). The unseen one casts the spells Disintegrate  (Save DC 27) on one target within range.

The unseen one 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 unseen one regains spent legendary actions at the start of its turn.

Teleport. The unseen one can teleport up to 300 feet to any unoccupied place it can see.

Spinning Strike. The unseen one can spin its weapon and any creature within 10 feet of the unseen one must make a DC 27 dexterity saving throw or take 42 (5d12+10) of any type of damage they choose or half as much on a successful one.

Destructive Power (Costs 2 Actions). The unseen one recharges its Disintegration ability and uses it on one target within range.

Call of the Hoard (Costs 3 Actions). The unseen one can bring forth one of its allies to its location and summon another unseen one, but it can only do this action once per day, but the unseen one it summoned is not limited from using its Call of the Hoard legendary action unless it already has. There are only 100 unseen ones in existence.