Black Knife Assassin

Assassinate. During its first turn, the black knife assassin has advantage on attack rolls against any creature that hasn't taken a turn. Any hit the assassin scores against a surprised creature is a critical hit.

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

Sneak Attack. Once per turn, the black knife assassin deals an extra 14 (4d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the assassin that isn't incapacitated, and the assassin doesn't have disadvantage on the attack roll. 

Magical Arms. If the black knife assassin is without its armor, it cannot cast Invisibility and if it's using a weapon other than a Black Dagger it losses its maximum hit point drain.

Spellcasting. The black knife assassin can cast Invisibility at will.

Multiattack. The assassin can make two attacks only one of which can be a Destined Death Slash.

Black Dagger. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 11 (1d4 + 9) slashing damage and the targets hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0 and it can't be revived short of a Wish spell.

Destined Death Slash. Ranged Spell Attack: +9 to hit, range 50 ft., one target. Hit: 13 (3d8) necrotic damage and the targets hit point maximum is reduced by an amount equal t the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0 and it can't be revived short of a Wish spell.