Cromm Cruich is an enormous grey dragon, or Mist Worm. He also appeared, due to Piet's shape changing powers, in a humanoid form. He says the race of dragons was dying before ours even evolved.
The Umbra Sprite summoned him to attack the heroes after Kevin was wounded by Emil's venom. Cruich was clearly the most powerful being the Umbra Sprite summoned; whenever his name is mentioned, the listeners are agape at the fact that he is on our plane. While Radu and Piet were sent along with him, they were just bystanders to his power. He effortlessly destroyed Sean Knight and broke into the heroes' apartment, to find them gone.
He returned to the Styx, where he lived in a lair containing enormous piles of baby skulls. He seems to have used their innocence as sustenance somehow, although it is unclear whether he literally ate them, or whether he just psychically fed on the innocence of the general population somehow.
When Kevin Matchstick invaded the Styx, he came upon him and confronted him. Once again, he made it clear how old and ineffable he was, saying that his teeth ached in his jaw before we ever dreamed up things like the Bat. At first, he refused to fight directly, sending instead his squires, which were raven-like birds. Matchstick dispatched them quickly. At that, Cromm Cruich blasted his mightiest fire at Kevin, but he deflected with the Bat, and leaped directly into his maw. Lightning exploded from his mouth and he died. Kevin jumped out of his mouth and flipped him over with one hand before leaving. So died the Umbra Sprite's mightiest creature.
Powers and Abilities:
Cromm Cruich has fire breath, which he can use an an enormous blast, or in very fine, controlled ways, more akin to spit or saliva. He was able to use the fire to disperse Sean Knight, a ghost, so perhaps his power has some supernatural element beyond just pure flame. In fact, the heat of his fingers was enough to burn Sean's forehead. However, Kevin Matchstick was able to stop his fire with the Bat relatively easily.
He was immune to Sean Knight's fear power, saying that he was older, more frightening, and more obscene, and so could not be scared by such a small spirit.
While he was obviously very powerful, he was temporarily stunned by the electrical output of a lamp that Sean transferred (and possibly amplified) into his body.