Shapechanger. The Shadow Sparrow can use its action to polymorph into a small sparrow, disguising itself as a normal sparrow, or back into its true form, which is medium. Its statistics, other than its size, are the same in each form, except for a +3 to it's AC when it's smaller.

Keen Hearing. The shadow sparrow has advantage on Wisdom (Perception) checks that rely on hearing.

Nimble Escape (sparrow form only). The Shadow Sparrow can take the (Disengage) or (Hide) action as a bonus action on each of its turns.

Soul Harvest. When someone of societal, prophetic, or yet-to-be determined importance dies within 120 ft of the shadow sparrow, it will use it's action to grab their soul with one of it's free talons.

Shadowfell Leap The Shadow Sparrow can freely move between the material plane and the shadowfell. While in it's nest, the Shadow Sparrow can open a gate to the shadowfell, large enough for medium creatures for 1 round.

Multiattack (True Form Only). In it's true form, the shadow sparrow can potentially make 3 attacks. 1 with it's beak and 2 with it's talons (if its carrying a soul, it can only make 1 attack with it's talons).

Beak. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 9) piercing damage.

Talons. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 23 (4d6 + 9) slashing damage. The target must also make a CON save (DC 12). On a failed save, the target takes 4d6 necrotic damage, or have as much on a success. 

The shadow sparrow can take 2 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn. The shadow sparrow regains spent legendary actions at the start of its turn.

Teleport. The shadow sparrow, drifts into the shadowfell for a moment, magically teleporting on the material plane, along with any equipment it is wearing or carrying and any creatures it has grappled, up to 60 feet to an unoccupied space it can see.

Energy Drain (Costs 2 Actions). Each creature within 30 feet of the shadow sparrow must make a DC 15 Constitution saving throw. On a failed save, the creature is overwhelmed with the negative energy of the shadowfell and takes a point of exhaustion.