You see a vermin swarm
Notes
Two of these will spawn when the The Sinister Hermit (Yellow) changes form to the The Sinister Hermit (Blue).
It looks the same as an Insect Swarm and a Berrypest.
Abilities
Melee (0-300?)
Self-Healing (250-350)Damage Taken From Elements
Physical 
Death 
Holy
Ice
Fire
Energy
Earth
Location
The Souldespoiler's room.
Behaviour
They fight in close combat.
Strategy
Kill them quickly before more of them spawn.