Patryn of Elvenshae said:There's something similar in Libris Mortis, but I don't remember the specifics - other than that it is a template that can be applied to Undead.
EDIT:
From the Return to Undermountain stuff on the WotC site:
Like I said - you have to be 11th level to become the lamest swarm (a swarm of rats). You get half damage from slashing and piercing, immunity to many single-target spells, and a vulnerability to area effects.Pielorinho said:Would there be any rules-unbalance to allowing this? I'd add the caveat that any rat who ventures more than 10' away from the main body of the swarm becomes a separate creature that wanders off, inflicting 1 point of damage to the swarm. This prevents the shift-and-scatter strategy that would otherwise be so tempting.
Daniel