Bertrum

Bonding. Bertrum can magically bond with one creature it can see, immediately after spending at least 1 hour observing that creature while within 30 feet of it. The bond lasts until Bertrum bonds with a different creature or until the bonded creature dies. While bonded, Bertrum and the bonded creature can communicate telepathically with each other at a distance of up to 100 feet.

Mimicry. Bertrum can mimic simple sounds it has heard, such as a person whispering, a baby crying, or an animal chittering. A creature that hears the sounds can tell they are imitations with a successful DC 15 Wisdom (Insight) check.

Eyes in Shadow. Bertrum has advantage on Wisdom (Perception) checks that rely on sight.

Beak. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 6 (1d4 + 4) piercing damage.

Call upon Murder (1/Long Rest). Bertrum magically transforms into a Swarm of Murder. Bertrum reverts to its true form if it dies or falls unconscious. Bertrum can revert to its true form using a bonus action.
While in a new form, Bertrum's stat block is replaced by the stat block of the Swarm of Murder, except Bertrum keeps its current hit points, its hit point maximum, and its Abilities.

Helpful. Bertrum is adept at giving well-timed assistance; Bertrum can take the Help action.