Trinimac lost his divine power and...

myskyrim26

So...
- First, Trinimac is described as "strongest of the Altmeri ancestor spirits", and "Of all the et'Ada who wandered Nirn, Trinimac was the strongest" and "Auriel's champion, Trinimac", and "Trinimac, Auriel's greatest knight" - all that in legends of Altmer, Chimer (Dunmer) and Orsimer (Orcs)
- Now, he interacts with a Heart of Lorkhan according to Altmer Monomyth: Auriel could not save Altmora, the Elder Wood, and it was lost to Men. They were chased south and east to Old Ehlnofey, and Lorkhan was close behind. He shattered that land into many. Finally 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.
- And after that Boethiah does some interaction with Trinimac, in most myths, eats and excreates him as Malacath. (This certainly happens after the interaction with the Heart of Lorkhan)

How so? Well, the answer might be a Heart of Lorkhan: Trinimac tried to destroyed it, but lost a part of his divine power. His weapon is a sword, so he interacted with a Heart with it - but we know that a Heart "needs" 3 instruments, Keening, Sunder, and Wraithguard. Trinimac had but one, and thus suffered. And Boethiah saw an opportunity to defeat the "strongest of the Altmeri ancestor spirits" - because he is not that strong anymore! He's weak and near-mortal now.
Please, discuss!

OtarTheMad

It's an interesting theory for sure and makes sense. I always thought that at some point Trinimac was held prisoner by Boethiah and lost some power since people stopped worshipping him and the weakened Trinimac grew angry over his peoples lack of worship and basically got corrupted into Malacath.

It'd be cool if in a single player TES game we could restore Trinimac's full power but I doubt it's possible, from what I read the bones you find in Malacath's realm is what is left of him. (Which supports a theory that Trinimac is dead and Malacath is just a weaker daedra who is claiming to be him).

I, personally, like the first theory that Malacath is a weakened and angered Trinimac. Probably weakened by lack of prayers and your theory.

RaddlemanNumber7

I think it always helps if you sort events into a timeline. This is how I see it...

Lorkhan was dismembered before or at the same time as the sundering. Lore puts Auri-el and the rest of the Aedra-to-be at the top of the Admantine tower, at convention, where Lorkhan was dismembered. Auri-el and Trinimac are named as being involved in trying to break Lorkhan's heart and failing. Auri-el is named as shooting Lorkhan's heart away.

The bit about Trinimac defeating Lorkhan after the sundering therefore seems to be just a bit of story teller's licence. A fiction.

All the stuff about "strongest of the Altmeri ancestor spirits", and "Of all the et'Ada who wandered Nirn, Trinimac was the strongest" and "Auriel's champion, Trinimac", and "Trinimac, Auriel's greatest knight" describes Trinimac on Nirn after the sundering. While Trinimac stayed behind on Nirn most of the other Aedra had fled to Aetherius, including the most powerful ones. So Trinimac may have been the strongest Aedra on Nirn, but it only means he is the largest among the small Aedra left behind on Nirn. Trinimac may also have lost strength by sacrificing himself to stay on Nirn after the sundering.

The business with Boethiah takes place much later. Just before Veloth and the Chimer start their exodus. Boethia is one of the biggest first-generation of Deadra. There's no reference in lore I know of that speaks of Boethiah being an aspect or a subordinate of anyone else. Trinimac seems to be a subordinate aspect of Auri-el, a fraction of Auri-el's strength. So, Boethiah always was bigger than Trinimac and, now that Auri-el and the rest of the big Aedra are no longer around to back up Trinimac, it is time for big Boethiah to show little Trinimac just who is the daddy now.

mnemoniclights

Keening, Sunder and Wraithguard were made by Kagrenac and are of clear Dwemer make is the problem. Kagrenac, the Tribunal and Dagoth Ur used the tools to abuse the heart and steal divine power from it.

Rake

Strongest? Maybe.
Stupidest? For sure.