I was just looking at a swarm of bats and I just don't get it. 5,000 in a 10x10 area?!? 5,000!!!
The average hp for the swarm is 13 (IIRC), which isn't very much even with the 50% dam from slashing & piercing weapons. It's quite possible for a fighter to deal that in one shot. 5,000 bats in...