He is described in The True Nature of Orcs as the strongest of the Aedra, the champion of the High Elven pantheon, in some places more popular than Auri-El. In The Anticipations, he is referred to as the champion of Auri-El himself.
The official view taken by the majority of the leading priests in Orsinium, put forth by king Gortwog gro-Nagorm, is that Trinimac still lives and Malacath is a separate entity, and impostor. A handful of Orcs within Orsinium and the majority of the Orsimer diaspora view Gortwog's beliefs as heresy.
The Monomyth states that "Trinimac, Auriel's greatest knight, knocked Lorkhan down in front of his army and reached in with more than hands to take his Heart. He was undone. The Men dragged Lorkhan's body away and swore blood vengeance on the heirs of Auriel for all time.
"But when Trinimac and Auriel tried to destroy the Heart of Lorkhan it laughed at them. It said, 'This Heart is the heart of the world, for one was made to satisfy the other.' So Auriel fastened the thing to an arrow and let it fly long into the sea, where no aspect of the new world may ever find it."
During the exodus of the Chimer in the Mythic Era, one of the most powerful Aldmeri deities, Trinimac, tried to stop the movement. The Daedric Prince Boethiah, mastermind of the movement, confronted Trinimac, and transformed him into the Daedric Prince Malacath. As a result, Trinimac's followers were changed as well into what are now known as the Orsimer (Orcs), some of which worship Malacath as their god. This is recorded in the The Changed Ones.
In The Anticipations, it is said that following Trinimac's demise, "the followers of Boethiah and Trinimac rubbed the soil of Trinimac upon themselves and changed their skins." The House of Troubles refers to Malacath as the reanimated dung of Trinimac.
The Changed OnesEdit
Of all the et'Ada who wandered Nirn, Trinimac was the strongest. He, for a very long time, fooled the Aldmeri into thinking that tears were the best response to the Sundering. They cried and shamed our ancestors, especially the feminine Altmer. They even took the Missing God's name in vain, calling his narratives into question. So one day Boethiah, Prince of Plots, precocious youth, tricked Trinimac to go into his mouth. Boethiah talked like Trinimac for awhile then, and gathered enough people to listen to him. Boethiah showed them the lies of the et'Ada, the Aedra, and told them Trinimac was the biggest liar of all, saying all this with Trinimac's voice! Boethiah told the mass before him the Tri-Angled Truth. He showed them, with Mephala, the rules of Psijiic Endeavor. He taught them how to build houses, and what items they needed to bury in the corners. He demonstrated the right way to wear their skin. He performed the way to walk to achieve an exodus. Then Boethiah relieved himself of Trinimac right there on the ground before them to prove all the things he said were the truth. It was easy for his new people to become the Changed Ones.