Unobserved Phenomena. Terumi's natural state when he lacks a body is an intangible spirit. In this state, he acts as though he is under the effects of the Etherealness spell, though he can see and hear in his current plane normally, and may choose to have other creatures perceive him (be it his body, his voice or both). Any creatures that can see him see a lanky black and green ghost with one eye and a red smile, which creatures with Truesight can see when looking at him normally. Additionally, he can always choose to affect creatures and objects on the ethereal plane as though it were physical.

Self-Observation. Terumi can create a physical form for himself, including clothes and anything he was carrying the last time used this trait. He can maintain this body for up to two weeks, either all at once or in several shorter spans, regaining charge for this ability equal to the time he isn't using it. When he stops maintaining his body, it and anything it was carrying disappears into green mist.

Shed Skin. If Terumi is reduced to 0 hit points or killed by any means that don't affect the Ethereal Plane, he may choose to be reduced to 1 hit point instead, but must immediately leave his vessel and/or stop maintaining his body with Self-Observation, and cannot use Self-Observation for two weeks.

Despised Destroyer. If Terumi is killed, if at least one person in the world has a deep hatred of him at the time of his death (as determined by the DM), his spirit reforms in 2 (1d4) days. Terumi reappears within 1 mile of where he was slain.

Legendary Resistance (3/Day). If Terumi fails a saving throw, he can choose to succeed instead.

Shadowy Serpent. While Terumi isn’t wearing armor, his AC includes his Intelligence modifier.

Cleaving Fang. Terumi's weapon attacks score a critical hit on a roll of 18, 19 or 20 against restrained or prone targets.

Force Eater. When Terumi lands a critical hit, he immediately recharges Distortion Drive.

Innate Spellcasting. Terumi can cast one of the following spells without requiring vocal or material components, using Intelligence as the spellcasting ability (spell save DC 23, spell attack +17):
At will: major image, detect thoughts, invisibility, dimension door
3/day each: modify memory, legend lore, dominate person, hold monster, plane shift
1/day each: weird, forbiddance, dominate monster, glibness

Multiattack. Yuuki Terumi makes three Ouroboros attacks. He can forgo two attacks to use Distortion Drive if it is available.

Ouroboros. Melee Weapon Attack: +15 to hit, reach 5 ft., 1 target. Hit: 21 (2d12 + 8) necrotic damage. The first time on his turn this attack hits a creature, that creature must make a DC 23 Strength save or be knocked prone.

Distortion Drive (Recharge 5-6). Terumi uses one of the following attacks.

Serpent's Cursed Sting. Melee Spell Attack: +15 to hit, reach 5 ft., 1 target. Hit: 92 (14d12) necrotic damage. Terumi turns Ouroboros' tip into an ornate dagger, and attacks with it. On a hit, ethereal snakes grasp at the target's body. This attack cannot crit.

Dungeon of Serpents. Terumi choose a point within 150 feet of him on the ground. All creatures other than himself within 60 feet of that point must make a DC 23 Dexterity saving throw as a portal opens up, causing a barrage massive of ethereal serpents to attack. On a failure, they take 52 (8d12) necrotic damage, or half as much on a success.

Unholy Wrath of the Basilisk (Nightmare Reaper Only). Terumi opens a portal behind himself, as a barrage of ethereal snakes assault his foes in a 40-foot cone, forcing them to make a DC 23 Dexterity saving throw, taking 98 (15d12) necrotic damage and be restrained until the end of their next turn on a failure, half as much damage and no restraining on a success. Terumi may then immediately target a creature within 5 feet of him who failed their saving throw, as a shadow of his original form surrounds him and creates a massive blade, dealing an additional 98 (15d12) necrotic damage. A creature reduced to 0 hit points by this damage dies instantly, as their body and all nonmagial objects they were carrying are obliterated from existence. Once Terumi completes this action, Nightmare Reaper immediately ends.

Nightmare Reaper (1/day). Terumi's body is wrapped in black and green ethereal energy. For one minute, he gains the following benefits:

• All necrotic damage Terumi deals will instead deal normal damage to creatures who resist necrotic damage, and it will instead deal half damage to creatures who are immune to it.
• Gleaming Fang may be repeated once when used.
• Venomous Bite deals 33 (5d12) necrotic damage instead of 20 (3d12)
• A target hit by Serpent's Cursed Sting must make a DC 23 Constitution saving throw. On a failure, they lose a spell slot of 5th level or lower, and Terumi immediately recharges his Distortion Drive.
• Dungeon of Serpents additionally causes all creatures who fail the saving throw to become poisoned. Creatures poisoned in this way take 14 (2d12) necrotic damage at the beginning of their turn, and must make a DC 23 Constitution saving throw at the end of each of their turns, ending the effect on a success.

Distortion Drive. The following bonus action uses Terumi's Distortion Drive charge.

Gleaming Fang. Brandishing Ouroboros, Yuuki Terumi rushes forward up to his movement speed in a straight line without provoking attacks of opportunity. All creatures who are within 5 feet of him during this dash must make a DC 23 Dexterity save, taking 26 (4d12) necrotic damage and be knocked prone on a failed save or take half with no knocking on a success.

Break Burst (1/combat). If Terumi takes damage, he may unleash a burst of power. All other creatures within 10 feet of him must make a DC 23 Constitution saving throw or be pushed 10 feet away from Terumi and knocked prone. Additionally, he immediately ends the prone, restrained, grappled or stunned condition on himself.

Distortion Drive. The following reaction uses Terumi's Distortion Drive charge.

Venomous Bite. When Terumi would be hit by a melee attack, he may use a reaction to force an opponent to make a Wisdom (insight) check against his Charisma (deception). If they fail, the attack misses instead, and the opponent is bound tight by ethereal chains, taking 20 (3d12) necrotic damage and being restrained until the end of their next turn.

Terumi 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. Terumi regains spent legendary actions at the start of his turn.

Snakebite. Terumi moves up to his movement speed, and then may immediately make a Grapple check. Terumi may use Dexterity (acrobatics) instead of Strength (athletics) for this check and later checks to maintain it.

Agonizing Fang. Terumi makes an Ouroboros attack. If it's a critical hit, he may spend an additional 2 Legendary Actions to immediately use his Distortion Drive action, then recharge it.

Crush Trigger (Costs 2 Actions). Terumi makes an Ouroboros attack. If it hits a creature, that creature gets -2 AC until the end of Terumi's next turn.

 Spectral Mind (Costs 2 Actions). Terumi casts one of his at will spells.