(Video Link)
"Monster Manual" is a song by the band Mixel Pixel. It tells the tale of a role-player's struggle with a particularly brutal Dungeon Master, who is throwing just about every creature in the Dungeons & Dragons Monster Manual at him. The video is animated by Dan Meth, whose work has been featured extensively on Neatorama.
via Popped Culture | Mixel Pixel | Dan Meth
What I believe you are thinking of is the Goddess of chromatic dragons, who's form is a dragon with five chromatic heads (one for each chromatic dragon type - black, white, red, blue, and black). Often goes by the name Tiamat, but has shown up under other names in different settings (Takhisis, for example).
Is is just me or was the animation reminiscent of Super Jail?