The question is simple, but the answer seems anything but.

If we are to take the tales of Trinimac's downfall at the hands of Boethiah as true, with the former being transformed into the Daedric Lord Malacath as a result, it raises the question (at least for me), why did Boethiah chose Trinimac specifically to make undone?

If it was possible for Boethiah to debilitate and/or imitate Trinimac, supposedly one of the mightiest Aedra, so completely, why did Boethiah not set his/her sights a bit higher and attempt to bring down the head honcho, Auriel/Akatosh, himself? What made Trinimac so susceptible to sabotage? Was Trinimac the only Aedra Boethiah could have brought down? If so, what about him made that possible?

I might suggest that, with fables surrounding the event of his fall, Trinimac may have grown complacent with his place in the cosmos or perhaps Boethiah thought he commanded more deference and reverence than he deserved, intervening too often and too directly in the affairs of the Aldmer, telling them how to view their place in the world and such. But, seeing as Malacath is a being who champions the spurned and ostracised and is fixated on the following a strict code of honour and ethics, I find it hard to believe that Trinimac was a being prone to hubris.

One thing that strikes me as particularly interesting and curious about the whole affair is the stories surrounding the rise of the Chimer. It is said that Boethiah claimed that Trinimac and presumably the other Aedra were wrong to tell the Aldmer that they should despair over their mortality and the existence that Lorkhan had bound them to. Was Boethiah's protest based on some characteristically atypical sense of justice? Or perhaps it was a vendetta brought on by some sense of fraternal duty to another divine Trickster? Or maybe (s)he just thought making accusations like that were a sure means to plant the seeds of chaos and confusion in Aldmer society?

Another line of questioning is, why did Boethiah bother with any of it? Was it just the anarchistic Prince's nature compelling him/her to screw with the Aedra and turn their followers against them or were Boethiah's motivations deeper than that? Did the Lord of Upheaval covet devotees? That doesn't strike me as his/her MO... Just what the heck was it all about?!

Trinimac was trying to halt the Velothi Movement of the Chimer. Boethiah, being one of the three Daedra behind the Velothi Movement, did not appreciate this. Therefore, He made Trinimac into the Pariah he is today, so that the Chimer could continue on their path. That's how I've always viewed his motives, anyway.

You get closest to the point here:

The shortest possible answer is: Lorkhan. The entire question of what Boethiah did relates to his role as a devotee of Lorkhan and a supporter of the principles that he inspired. Trinimac personally killed Lorkhan and cast his heart out to Morrowind; Boethiah firstly holds a vendetta for this, and secondly, Boethiah is one of the primary ancestors revered as part of the Velothi movement, which supported moving into Tamriel and abandoning the Altmer/Aldmer principles of conservation and preservation. Trinimac, as knight of the Elven god Auri-El and staunch opponent of all seen as Lorkhanic or Mundrial taint, opposed this movement, and Boethiah swallowed/defied/disfigured him in order to teach the Chimer and defy Aldmer orthodoxy.

Interesting and informative responses. Thanks, guys. Though, I suppose I'm still wondering why Boethiah was invested in the Velothi movement at all. When encountered in Skyrim, Boethiah seems to be quite resenting and disdainful of those who revere him or would seek his favour. Is it because, as a naturally defiant creature, (s)he sees any dissent as good dissent, even if it means (s)he is deified in the process? If so, it does offer an insight into Boethiah's character and shows that perhaps (s)he does not view sycophancy as completely contemptible, particularly if it is directed towards him/her.

Trinimac/Malacath is Keeper of the Sworn Oath, Boethiah is a lord of lies and deception. They were fated to do combat at some point, especially considering Boethiah's love for battle and competition. Plus, Trinimac was the glue holding all of the Aldmer together as one ideology. Being a Prince of anarchy, it makes perfect sense as to why Boethiah would want to disrupt that order.

I still don't understand how this could go down exactly. Did the gods just walk the earth? Were they massive?

It was the Dawn/early Merethic era. Lots of things were nebulous back then from the perspective of the present.

A very sensible conclusion, Malak! The lack of open animosity between the two does seem quite peculiar, but, in line with your point, it would perhaps make sense that Malacath, a being who holds oaths above all else, would be more resenting of former comrades who had forsaken him in his fall from grace, rather than the enemy who dragged him down. As I think you said in part of your Trimalarkay compilation, someone like Trinimac/Malacath would likely prefer an open enemy to a fickle friend.

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The events of the Dawn Era~ do all seem very abstract and vague, Mr. Crab, both in the lore and the writing. Personally, I'm split between two interpretations. One, thinking that much of the mythology is romanticized and believing that Trinimac would have been limited to communicating with his followers in a similar way to how Aedra seem to contact their devotees in the Fourth Era, through select priests or 'prophets' like Dibella's Sybil and Boethiah simply had to whisper sweet nothings into the right, impressionable ears. Or, two, the Aedra did indeed walk among mortals in that era, titanic figures with an ethereal aura akin to the gods in Disney's Hercules (but only for a limited time before they inevitably became increasingly intertwined with the the physical plane that was composed of their essence) and the mantling process might have involved some kind of arcane fusion of essences until the instigating host chose to relinquish their vessel and Boethiah revealed himself to a horrified throng of spectators like some masterful pantomime villain or Jojo's Dio (https://www.youtube.com/watch?v=woa9SVDQWZ0)!