B2

Assassinate. During its first turn, B2 has advantage on attack rolls against any creature that hasn't taken a turn. Any hit B2 scores against a surprised creature is a critical hit.

Evasion. If B2 is subjected to an effect that allows it to make a
Dexterity saving throw to take only half damage, B2 instead
takes no damage if it succeeds on the saving throw, and only
half damage if it fails.

Sneak Attack (1/Turn). B2 deals an extra 21 (6d6) damage when
it hits a target with a weapon attack and has advantage on the
attack roll, or when the target is within 5 ft. of an ally of B2 that
isn't incapacitated and B2 doesn't have disadvantage on the
attack roll.

Innate Spellcasting. B2's innate spellcasting ability is Charisma
(spell save DC 13, +5 to hit with spell attacks). It can innately
cast the following spells, requiring no material components:
At will: Darkness, invisibility

Aversion to the Golden Blade. If the B2 takes damage from the
Golden Blade, it has disadvantage on attack rolls and ability
checks until the end of its next turn.

Cunning Action. On each of its turns, B2 can use a bonus action
to take the Dash, Disengage, or Hide action.

Fear Aura. Any creature hostile to B2 that starts its turn within
10 feet of B2 must make a DC 19 Wisdom saving throw, unless
B2 is incapacitated. On a failed save, the creature is frightened
until the start of its next turn. If a creature's saving throw is
successful, the creature is immune to B2's Fear Aura for the
next 24 hours.

Multiattack. B2 makes four attacks and casts a spell.

+3 Shortsword. Melee Weapon Attack: +12 to hit, reach 5 ft., one
target. Hit: 11 (1d6 + 8) piercing damage.

Uncanny Dodge. an attack that hits B2 deals half damage.