In spite of talk to the contrary, magic is a tangible force. Regardless of the specifics behind where it comes from, energy of some kind rips through reality to create some sort of fantastic effect. For the most part, this energy is burned away in the process of a spell - a fireball or magic missle disspates with casting. However, occasionally a spell will leave some sort of residue.
Prismatic spells are especially bad about this. Any monster whose lived long enough can point out the tell-tale color splatters along dungeon floors and walls that indicate you have some rainbow throwing asshole mucking around your home. The trouble with these multi-hued messes don't end there however, as there are a handful of these pools that retain just a bit of magic.
Sometimes, bugs will get to them. And what happens when an ant or a bee returns to its nest with rainbow juice from its latest sortie? Well, if they survive the initial exposure, a Prismatic Swarm, thats what.
Said swarm becomes hyper-charged with strange magic and usually ends up looking like a rolling or flying cloud of stained glass from far away. Once close, thier actual properties become pretty obvious. The creatures that make up the swarm are now sustained as well as driven mad by that sorcery, and attack any mobile target that crosses thier path.
HD: 3 (Special, see below)
Saves as: Fighter 2
Special: A Prismatic swarm actually engulfs a potential victim instead of attacking them normally. A target may make a saving throw in an attempt to get out of the way - if they have been engulfed however, they are hit automatically. They may make another saving throw to escape on subsequent rounds, or try some other means of escape. As a note; fire will not disperse the swarm, but submerging oneself in water will cause them to seek other targets.
Whenever the Swarm "hits" someone, roll a 1d8 and consult the following table;
1d8 Color Effect
1 Red 1d8 (Fire)
2 Orange 1d4 (Acid, does 1 HP of Damage each round in till cleaned off)
3 Yellow 1d6 + 2 (Electricity)
4 Green Save versus Poison; Failure equals death (Poison)
5 Blue Save versus petrification or turn to stone
7 Violet Save Versus spell or vanish to another plane/dimension
8 Oh F*ck Roll twice on table, re-roll "8's", apply both effects
Due to the nature of a swarm, normal attacks only do 1d3 points of damage to it. Attacks that effect a large area will do normal damage.