The Dumahim were one of the five vampire clans that appeared in Soul Reaver. They were the descendants of the lieutenant Dumah. When Raziel returned from his hellish execution in the Abyss, the first vampires he encountered were Dumahim fledglings. Raziel did not immediately recognize them - he called them "foul, scuttling beasts." He was shocked and disgusted to learn that they, like him, were descended from Kain.
At the time of Raziel's execution, the vampires of Kain's empire still had a humanoid shape. During his centuries-long absence, the clans had evolved far beyond what he remembered. The Dumahim had become heavily armoured. They also had long lashing tongues that they could flick out at their enemies; powerful enough to injure Raziel, and probably capable of killing a human. The fledglings appeared to have greenish skin on the faces and limbs, but the adults were blue.
The Dumahim were the most common vampires in Soul Reaver. The other clans largely kept to their own territories, but the Dumahim had spread out across Nosgoth. Raziel found that they had overrun his desolate former stronghold, amongst other places. When Raziel set out to find Dumah, he discovered the reason for their nomadic behaviour.
Raziel visited the Dumahim city (sometimes called 'Ash Village' in non-game sources)¹, and saw that a massacre had taken place there, long ago. Dumahim corpses were scattered everywhere, slowly decomposing to skeletal remains. Raziel suspected that Kain might have been responsible, but he was corrected by the Elder God. The devastation in the ruined city was Dumah's fault.
Dumah and his clan had been too arrogant, failing to acknowledge the small threat posed to them by Nosgoth's humans. They paid a high price for their complacency - their territory had been attacked by human vampire hunters. The surprise attack had caused heavy losses to their number, and permanently incapacitated Dumah. Deprived of his leadership, those Dumahim who survived had been reduced to scavengers.