Boris

Assassinate. During its first turn, the 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.

Clockwork Immutable Form. Boris is immune to any spell or effect, other than its disassembly trait, that would alter its form.

Magic Resistance. Boris has advantage on saving throws against spells and other magical effects.

Psychic Susceptibility. If Boris takes psychic damage, it has disadvantage on attack rolls, saving throws, and ability checks until the end of its next turn.

Sneak Attack (1/Turn). Boris deals an extra 10 (3d6) 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 Boris that isn’t incapacitated and Boris doesn’t have disadvantage on the attack roll.

Standing Leap. Boris’s long jump is up to 20 feet and its high jump is up to 10 feet, with or without a running start.

Multiattack. Boris makes two piecing arm attacks.

Piercing Arm. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) [piercing] damage. 

Helpful. Boris can use the Help action as a bonus action.

Cunning Action. Boris's agility or quick thinking allows it to act speedily. On its turn in combat, it can take the Dash, Disengage, or Hide action as a bonus action.