Akikage (5e Creature)

Medium undead, lawful evil

Armor Class 15 (natural armor)
Hit Points 110 (20d8 + 20)
Speed 40 ft.

Skills Acrobatics +7, Stealth +7
Proficiency Bonus +3
Damage Resistances bludgeoning, piercing and slashing from nonmagical attacks
Damage Immunities cold, necrotic, poison
Condition Immunities charmed, exhaustion, frightened, grappled, paralyzed, petrified, poisoned, stunned
Senses darkvision 60 ft., passive Perception 11
Languages Common
Challenge 7 (2,900 XP)

Death Strike. If the akikage is visible and hits a creature with a phantom blade attack, it can use a bonus action to force the target to make a DC 12 Constitution saving throw. On a failed save, the target's heart freezes in its chest, reducing its hit points to 0. On a successful save, the target takes 35 (10d6) cold damage instead.

Incorporeal Movement. The akikage can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Invisibility. The akikage is invisible. As an action, it can choose to be visible until the end of its next turn.

The akikage, or phantom ninja, is the spirit of a ninja assassin who died while stalking an important victim. In life, the akikage was obsessed with duty and discipline. Now it cannot rest in its grave until it has successfully completed its final mission.
The akikage is an incorporeal spirit that is rarely visible. When seen, the creature is dressed all in black. It wears tight gloves and boots as well as a mask and hood that hide all traces of its former humanity. The akikage cannot speak but seems to understand the words of those around it regardless of the language in which they are spoken. The akikage talks only when magically compelled to do so.
This grim spirit seeks no company save that of the person it is tasked to eliminate and will attack anyone who tries to stop it. If the akikage successfully completes the mission that it failed in life its spirit is released from its curse of undeath. At this point, the spectral menace dissipates and vanishes for good.
Tales are told of akikage that are forced to serve a living master. This enslavement usually occurs when an evil spellcaster uses malign magic. It is also possible to gain control of a phantom ninja by discovering who the intended victim of the spirit assassin is and killing him before the creature can strike. If this is done and a control undead spell is promptly cast upon the body of the akikage’s target, the phantom ninja will become an ansasshia, doomed henceforth to obey the caster of the spell. While under the influence of another, these undead assassins carry out their orders promptly and efficiently. While they might yearn to be free of their master’s will, their devotion to duty and honor makes it impossible for them to disobey or betray him in any way.

Undead Nature. An akikage doesn't require air, food, drink or sleep.