Swarm is a skill located in the Animal Companions (which can be found in the Warden skill tree). Swarm is a base skill, and can be morphed into Fletcher Infection or Growing Swarm.
Swarm deals the following types of damage: Magic Damage. Which means that it gets buffed by the following blue champion point trees: Elemental Expert, Elf Born, Spell Erosion, Thaumaturge. Also the following red champion point trees protect against this skill: Elemental Defender, Thick Skinned.