Fell Bats

Fell Bats is a Vampire Counts melee war beasts unit in Total War: Warhammer. In Rise of the Tomb Kings, the Followers of Nagash faction also has access to these. Fell Bats fall upon their enemies in a seething mass; in numbers, they become overwhelming.

Description
With a body as long as a grown man, Fell Bats are a fearsome sight. They are darker than midnight and silent as death, even when in full flight. In fact, the only noises that Fell Bat makes on the hunt are horrible gobbling slurps when it sinks its distended mouth into living flesh. In truth, a Fell Bat bears as much resemblance to an ordinary bat as a maddened lion does to a domestic cat. Those who have encountered them, and lived, tell how they hunt with unerring accuracy, swooping down to knock Knights from their saddles or pick off lone warriors unawares. Vampires view Fell Bats much as a Bretonnian Lord might his prized hunting falcons. Precious sweetmeats are offered to Fell Bats from the ramparts of the Vampire's fortress, but to harness an entire flock, a grander sacrifice must be made. A fresh victim, belly-slit and screaming, or a terrified messenger sent to run into the night with his eyes put out is more to their taste. Once they have the scent of gore, they will fall upon the enemy battle line with bloodthirsty intent.

Useful for picking off siege engines and archers
Ideally you'll want to have a group of 3 to 4 units of fell bats and make them attack siege or archers all at once. as far as I can tell they don't block each other from attacking units quite as much as other units do so they all get their hits in and can wipe these units out really quick. the tricky part is getting them to kill the siege quick enough and retreat back up into the air before the enemy sends armored units to flank the bats. the bats can't do heavily armored units at all. this makes them less useful against non archer units however they can be used to chase routing units to end of the map so they don't come back.

Excellent Screening Unit for Vargheists and Terrorgheists
While they are effective on their own for picking off artillery, routing units, and most ranged units (ie. not dwarfs), they also have a lot of value in screening damaging dealing flying units. They can't take focused or sustained missile fire very well, but it's preferable to have a screening unit of bats get shot to pieces and get your Vargheists/Terrorgheists into the enemy backline intact. Bats have very high melee defense so if you manage to get a unit or two of them into the enemy backline along with Vargheists/Terrorgheist, they will last a long time in melee and greatly interfere with your enemies attempts to pin and destroy your damage dealing flyers as they run amok. It's much cheaper and generally more effective to pair a unit of bats with each heavy flying unit than just fielding more Vargheists/Terrorghiests.