This inscription can be found on the Statue of Grenth:
Came then Desmina, scorned and exiled by her people. And in her misery and wretchedness, did Desmina curse the gods for abandoning all who, like her, admired power and ambition.
And asketh she, "Where is the god to whom I may give my undying devotion? Where is the God to whom I may beg revenge against those who scorn me?"
And rumbled then the earth from far below, and with a terrible groan, split open. The ground grew white with frost and ice, and from forth the frozen earth spilled the rotted, skeletal minions of Grenth. Appeared then the god, and with bony hands outstretched, welcomed the girl into His fold. Saith he, "I am your god. Follow where I lead, come whence I call, and the rotted corpses of the dead will be yours to control." And swearing allegiance in life and beyond, did Desmina thence become the god's first follower.
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 regain what Grenth took from him by aiding Abaddon and by sending his minions to invade the Underworld.
It is suspected that Grenth may have replaced Abaddon as the god of water, however, this is never outright stated, and the water portfolio may have been divided between several gods.