Assassinate. During his first turn, Kayn has advantage on attack rolls against any creature that hasn't taken a turn. Any hit Kayn scores against a surprised creature is a critical hit.

Evasion. If Kayn is subjected to an effect that allows him to make a Dexterity saving throw to take only half damage, Kayn instead takes no damage if he succeeds on the saving throw, and only half damage if he fails.

Assassin's Mark. Once per turn, Kayn deals an extra 14 (4d6) damage when he hits a target with a weapon attack.

The Darkin Scythe. Kayn deals an extra 8 (2d6) necrotic damage. If Kayn drops below 20% of his maximum hit points he must succeed a DC 18 Wisdom saving throw. On a failure Rhaast, The Undoing kills him and takes over his body. On a success, Kayn heals until he has 50% of his maximum hit points.

Legendary Resistance (2/Long Rest). If Kayn fails a saving throw, he can choose to succeed instead.

Multiattack. Kayn makes 2 Darkin Scythe attacks. Kayn can substitute a Blade's Reach for one of his attacks.

Darkin Scythe. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 5) slashing damage and 8 (2d6) necrotic damage.

Reaping Slash. Kayn dashes forward 5 ft. in a line then spins while cutting all enemies within 5 ft. of him. Targets must make a DC 16 Dexterity saving throw, taking 18 (4d6 + 5) slashing damage and 8 (2d6) necrotic damage on a failed save, or half as much damage on a successful one.

Blade's Reach. Melee Weapon Attack: +9 to hit, reach 10 ft. line. Hit: 13 (2d6 + 5) slashing damage plus 8 (2d6) necrotic damage. Creatures hit must make a DC 16 Constitution saving throw or have their movement reduced to 0 for 2 rounds. Affected creatures can reroll this save at the start of each their turns ending the effect early on a success.

Umbral Trespass (Recharge 6). Kayn becomes shadow and seeps into a creature he has damaged in the last 3 rounds. At the start of his next turn he violently tears out of the target. The target must must make a DC 16 Constitution saving throw, taking 18 (4d6 + 5) slashing damage and 8 (2d6) necrotic damage on a failed save, or half as much damage on a successful one.

Shadow Step. Kayn uses the Shadow Technique to enshroud himself in shadow. While in this state Kayn can move through solid objects unimpeded and see through objects he is within, if he ends his movement within an object he must use his bonus action at the start of his next turn or be shunted out of it and take 16 (4d8) force damage.

Uncanny Dodge. When an attacker that Kayn can see hits him with an attack, he can use his reaction to halve the attack’s damage.

Kayn can take 3 legendary actions, using the options below. He can take only one legendary action at a time and only at the end of another creature’s turn. Kayn regains spent legendary actions at the start of his turn.

Found You! Kayn makes a Perception check.

No Escape... Kayn uses Shadow Step and moves up to his speed toward a creature.

Attack. Kayn makes a Darkin Scythe attack.

Reap (Costs 2 Actions). Kayn uses Reaping Slash.

Hush... (Costs 3 Actions). Kayn recharges and uses Umbral Trespass.