Distilled of pure awesome

Lemming Swarm
Fine Magical Beast
Hit Dice: 12d10 (66 hp)
Initiative: +2
Speed: 15 ft. (3 squares), burrow 15 ft.
Armor Class: 20 (+8 Size, +2 Dex), touch 20, flat-footed 18
Base Attack/Grapple: +12/-
Attack: Swarm (3d6+curse)
Full Attack: Swarm (3d6+curse)
Space/Reach: 10 ft./0 ft.
Special Attacks: Distraction
Special Qualities: Swarm Traits, Dark Vision 60', Immune to Weapon Damage, Lemming Curse
Saves: Fort +8, Ref +10, Will +6
Abilities: Str 1, Dex 15, Con 10, Int 2, Wis 14, Cha 10
Skills: Hide +18, Listen +7, Spot +7, Swim +4
Feats: Alertness, Ability Focus (Lemmings Curse), Endurance, Run, Cold Endurance
Environment: Temperate or Cold Forest, Hills, Mountains, or Plains
Organization: Solitary, Cluster (3-6 Swarms), or Hive (10-20 Swarms) plus converts (CR of converts should be equal to 1/2 total CR of the swarms)
Challenge Rating: 7??
Treasure: None
Alignment: Usually Neutral
Advancement: -
Level Adjustment: ----

These look pretty much like any other Lemming. Little fuzzy critters. Surrounded by large, sometimes not so fuzzy critters with blank expressions on their faces...

Lemmings Curse (Su): Anyone subjected to the Lemmings Swarm attack must make a DC 18 Willpower Save (Save DC is Charisma based) or also be subjected to the Lemmings Curse. If it makes the Save it is immune to this swarms curse for 24 hours. If it fails it's mental stats become the same as the Lemming Swarms, and it believes itself to be one of them. The victim will do it's absolute best to defend the swarm. However it will possibly lose some feats, spells, class abilities, etc due to it's new mental stats. Since it now has the mind of a Lemming it no longer understands any language, and cannot communicate. Nor does it remember it's old life or friends. The curse lasts until the poor fool is eaten, or some kind soul casts Remove Curse/Miracle/Wish, etc.

Combat: The Lemmings aren't interested in wiping opponents out as having them join the horde. If soemthing is immuneto their Curse, they'll have the converts take him out back, so to speak...