Sluagh inhabited the Spectral Realm in Soul Reaver, Soul Reaver 2 and Defiance. They were "the scavengers of the Underworld", and they fed on unfettered souls. They were Raziel's competition when he was searching for sustenance in the Spectral Realm; largely pack hunters and quite agile, but not very intelligent. Like the Vampire Wraiths of Soul Reaver, a defeated Sluagh (one with no soul energy), would become transparent, and Raziel could consume it.
The Sluagh varied a bit between the three games. In Soul Reaver - again, like the Vampire Wraiths - they had coloured halos or auras, which indicated their level of health. A blue halo indicated that a Sluagh had a lot of soul energy; green, yellow and orange indicated that it was weaker. A red halo meant that a Sluagh's soul energy was almost depleted. If Raziel encountered a lone Sluagh, or one with a red halo, it would usually flee from him rather than fight. However, left alone, they could devour souls to regain their strength.
In Soul Reaver 2, the Sluagh seemed bolder. They would attack Raziel even when alone, only fleeing when they were defeated and transparent. Also, Soul Reaver 2 featured large beast-like Greater Sluagh that ran on all fours.
In appearance, the Defiance Sluagh seemed somewhere in-between the two Soul Reaver 2 types, but they notably engaged in cannibalism to restore themselves. As well as consuming souls, one transparent Sluagh would greedily devour another to survive.
Unlike Raziel and the Defiance Archons, the Sluagh were not agents of the Elder God. Although they devoured souls, they didn't return that energy to the Elder and the Wheel of Fate. This was indicated in Soul Reaver when the Elder said "their feral hunger has claimed countless souls - spirits who now shall never find their rest". Also, in Defiance, Raziel had to clear a Spectral cavern of Sluagh "vermin", as proof of his renewed obedience to the Elder God.[1,3]
- [SR1] The Elder God to Raziel: "Sluagh, the scavengers of the Underworld. Their feral hunger has claimed countless souls - spirits who now shall never find their rest."
- Soul Reaver 2: Prima's Official Strategy Guide (ISBN 0761532374)
- [DEF] Raziel (referring to Sluagh): "You want this room cleared of its vermin."
Elder God: "If you wish to leave it."