Grenth

Grenth is the god of death and cold/water. He is the patron god of Ritualists, Necromancers, Hydromancers and the lord of the Underworld.

This inscription can be found on the Statue of Grenth:

Grenth is the darkest of the Gods, and depicts his dark image with great efficiency. Although his body is that of a human, he bears the skull of a fallen beast. Grenth is often seen with minions crawling for his acceptance, for his followers are usually fanatic in his teachings. As revealed in The Necromancer's Path quest, Grenth promotes the casting off of illusions and 'the gilded trappings of sanctity', and encourages the perception of the world as it truly is. This fits with his role as judge to dead souls, and perhaps opposes him to Lyssa, goddess of deception.

Grenth wasn't always the god of death, and it is unclear what Grenth was before he became a god. The previous god of death was Dhuum, but Grenth destroyed his tower and took control of the Underworld. Dhuum now seeks to gain what Grenth took from him by aiding Abaddon and by sending his minions to invade the Underworld.

Grenth also replaced Abaddon as the god of water. Dialogue in Nightfall confirms Grenth was not god of death at the time of Abaddon's banishment, meaning it is possible Dhuum took up the mantle as god of water before he was overthrown by Grenth.