In Soul Reaver, Dumah was one of Kain's six vampire lieutenants. He was present at the meeting where Raziel revealed his newly-grown wings, and when Kain ordered Raziel's execution, it was Dumah (along with Turel) who carried him to the Abyss and threw him in.
Raziel did not forget Dumah's betrayal. When he returned from the Abyss as a Reaver of Souls, he hunted all of the lieutenants - his former brethren. They had all abandoned him to Kain's unjust will, and none had protested against the irrational sentence. In Raziel's mind, Dumah and Turel were especially guilty for their involvement. Exploring the Tomb of the Sarafan, Raziel was shocked to learn of their shared Sarafan heritage, but the revelation did not blunt his need for retribution.
When Raziel arrived at the Dumahim territory, he was surprised to see it littered with decomposing vampire corpses. Dumah's arrogance had caused him to overlook the small threat posed by Nosgoth's humans. At some point, they had stormed his city (sometimes called 'Ash Village' in non-game sources)¹, and slaughtered much of his clan. Raziel saw that the humans had already exacted some measure of vengeance for him: Dumah himself had been humiliated, having been 'killed' by the human invaders.
Dumah had been chained to his throne and impaled with three huge stakes. He still existed in the Spectral Realm, and Raziel could speak with him there (or even try to fight him there, although his efforts would be in vain). Dumah could only be killed properly in the Material Realm, so Raziel reanimated him by removing the stakes from his body. Dumah thanked his fallen sibling. His ego was evident - Dumah believed that his time in limbo had honed his strength. He claimed to be more powerful than Kain.
Having spent so long in the Spectral Realm, Dumah was technically a Revived Vampire, a Vampire Wraith, who upon re-inhabiting its body, retains advantages it gained in the Underworld. Dumah had their long-range soul-sucking ability.
Physically, Dumah had evolved into an armoured titan, but as Raziel said: "Even the strongest vampire is vulnerable". Raziel led Dumah into the ruined city's furnace and incinerated him. Dumah's soul endowed Raziel with the 'Constrict' ability. It is worth noting that this hard-earned skill - used to turn dials, stun enemies or kill humans - has not appeared in any subsequent game. The abilities gained from Melchiah, Zephon and Rahab have.
In Soul Reaver 2, Raziel saw Dumah's portrait in the Sarafan Stronghold. He was pictured with a purple, demonic creature. When Raziel travelled back further in time, he encountered the living Dumah. Five hundred years before the events of Blood Omen, the human Dumah had been a Sarafan Inquisitor. He was a member of the hunting party that murdered Janos Audron. He stole the Reaver as they fled from Janos' aerie.
Raziel pursued the Inquisitors back to the Sarafan Stronghold. Motivated by grief, anger and self-loathing, he killed them all with the Reaver (which Moebius had allowed him to reclaim). Dumah had been paired up with Rahab, and Raziel fought them in the main hall of the Stronghold. It became clear that the purple demon shown in Dumah's memorial portrait was in fact Raziel - artistic license had distorted his image. Dumah was buried in the Tomb of the Sarafan, and approximately a thousand years later, he was resurrected by Kain as a vampire.
Although there is no conclusive evidence, Dumah is commonly believed to be the third lieutenant raised by Kain, after Raziel and Turel. There are a couple of indicators to support this idea. He had the dubious honour of actively participating in Raziel's execution. Also, he was the final 'boss' in Soul Reaver (except Kain), and game convention dictates that the strongest enemies are usually faced last.