Bat swarm Warhammer fantasy battle 8th edition Vampire Counts overview
What are bat swarms?
As the name implies bat swarms are just that swarms of smaller bats (fell bats are much larger then normal bats) they are often found in places where vampires and necromancers dwell or where they seek out cadavers and spirits to swell their undead armies.
Some vampires find they have an affinity with beasts of the night such as bats and wolves and can summon and command them . The more powerful wizards amongst the vampires and necromancer summon artificial darkness to cover there armies and such conditions see the sky darken with bats further adding to the doom and gloom that hangs over the armies of the vampire counts.
In the game
In the game bat swarms are a special choice swarm for vampire counts in 8th edition warhammer fantasy battle.
Differing from 7th edition bat swarms are now a special choice in 8th edition rather then a limbo choice that counted towards nothing (as direwolves also were). The cloud of horror special rule has also had a revamp (vampire counts pun) as it now causes any enemy unit in base contact to gain the always strikes last special rule.
They also gain the hover special rule which basically means they cannot make a flying march move, this is no great loss as vampire counts units can only march now anyway if the general is nearby.
Pros and cons
The only real pros going for bat swarms is that they fit a theme well, and they cause any enemy unit in base contact to gain the always strikes last special rule which can occasionally be used to good effect.
Sadly in most situations the cons outweigh the pros for bat swarms they are relatively expensive for what they are have a terrible stat line and die rather swiftly against any foe once in combat. This will also drag your combat resolution down. They also have the hover rule which whilst not a massive loss but it does means they cannot march fly even if your general is nearby
The only real tactical use (and it is risky) for bat swarms is to force an enemy you are hopefully going to wipe out to strike last, therefore giving you the chance to do so. This for example could be a combined charge from a Vampire Lord and a bat swarm against a unit of always strikes first high elves, it negates the high elves always strikes first special rule allowing you Vampire Lord to strike them first.
Ideally you need to wipe the unit out as a wily opponent will direct attacks at the bat swarm as it is easy to kill and also provides easy combat resolution. Losing by a large amount will also spell disaster to your Vampire Lord or other unit as they suffer further wounds from crumbling.