Not much is known about Trinimac before his unmaking. What we do know is that it was he who killed Lorkhan and took his Heart. We also know that, according to some sources, he was considered the most powerful et'Ada to ever walk upon Nirn, and that some sects of Aldmer held him in higher regard than even Auri-El. He was also a guardian of the Aldmer, protecting them and their way of life from threats both foreign and domestic. But what is his sphere, exactly? We must look to Malacath for answers. Like Mithras, Malacath is a god associated with oaths, hence his moniker 'Keeper of the Sworn Oath and the Bloody Curse'. Malacath, like Mithras, takes deception very seriously, and it is quite apparent in his affiliated quest in Skyrim. He's willing to curse an entire family of his people, all because of the actions of one dishonest chieftain. Reminds me of a passage from the "Hymn to Mithra", which says "The ruffian who lies unto Mithra brings death unto the whole country, injuring as much the faithful world as a hundred evil-doers could do." This extreme attitude and behavior towards lying certainly makes it understandable why it was Trinimac who killed Lorkhan. That being said, Trinimac was undoubtedly a god of truth, even before he became Malacath. But there might be more to his sphere than that. Let's refer to a passage or two about Mithras and see if we can connect some of his roles with Trinimac.
And another...
Here we see Mithras variously referred to as a god of oaths, justice, light, the sun, and war. He also appears to be affiliated with agriculture through cattle-herding, and it just so happens that he is a patron of kings and the Roman legions. Those all sound awfully familiar, but look closely at the last two roles I mentioned. Many people believe Trinimac to be the equivalent of Zenithar within the Altmeri pantheon. This idea came about with the release of "Shor son of Shor", which equates Trinimac with Tsun to the point where both gods switch places. I've often challenged this belief with the mentioning of Xen within the Altmeri "The Heart of the World". He is named seperately from Trinimac within the text, so how could Trinimac be his equivalent within the pantheon? And then there is the issue of Orkey. My belief was that Arkay and Zenithar shared a similar relationship to one another as the one between the Time God and Space God, due to the overlap between them. For example, both gods are considered agricultural deities among the Redguards, and Tsun serves a similar purpose to Tu'whacca as a psychopomp, a conductor of souls into the afterlife. That's why I believe Trinimac and Tsun switched places, because their fathers, Ald and Shor, had switched places.
But there's another problem: Stuhn.
In "Shor son of Shor", he and Tsun are constantly shifting about throughout the text, possibly due to all the overlap between their spheres. They're twins for [censored]'s sake! So does this mean that Trinimac is Stuhn by extension? Let's refer back to one of the roles of Mithras. He is a god of justice, as well as a patron of rulers and the Roman legions. Is Stendarr not a god of justice? Is he not a patron of magistrates and rulers, AND the Imperial Legion? And thanks to the introduction of the Vigilants of Stendarr, he is also a god of light, just like Mithras. ESO heavily expands upon Stendarr's light through multiple texts, such as "Prayer of the Resolute", "Rituals of the Harmonious Masters", and "Aura of the Righteous". I think it also bears mentioning that, according to ESO, the Redguard gallants (knights) are patroned solely by Stendarr. Let's not forget that Trinimac was THE god of knights in his time, so there is a lot in common between he and Stendarr.
At last, we come to Arkay. But before we begin with him, let's go over a few more things concerning Mithras, particularly his role as a sun god. As mentioned above, Mithras is famed for slaying the cosmic bull that gave birth to all life. This act was necessary so that man could successfully inhabit the earth. In doing so, the bull's body and blood gave rise to all plant life and useful animals within the world, and ushered forth the Good Rains. I don't know about you, but this sounds a lot like Lorkhan's sacrifice, followed by Kyne's tears. Not only did Mithras set in motion the fertility of the planet in slaying the bull, but he also displayed his superiority over Ahura Mazda, thus earning him the titles of Sol Invictus, or 'invicible sun', and kosmokrator, or 'cosmic ruler'. Here's a few passages going over his duties as sun god and kosmokrator.
Not finished...
One more...
Now we can finally go over Arkay. As many of you know, Arkay is known as the 'Lord of Seasons' due to his association with time, and he is also referred to as the 'Lord of the Wheel of Life', according to "The Consecrations of Arkay". This puts him in control of the cosmic order of the Aurbis, which is easily identified with Mithras and his roles. You're probably thinking, "But Arkay isn't a sun god!" Well, let me ask you this: what is the sun, if not a massive soul-gate from which all souls pour forth from Aetherius? If Arkay is the one in charge of maintaining this soul-gate, then that makes him a sun god. Just not THE sun god. I think you guys/gals should take a look at an Amulet of Arkay, too. The symbol used looks VERY reminescent of a sun. But anyway, Arkay's power over souls is reflected in Orkey, who uses his power to shorten the lifespans of the Nords via curse. And then there is the story of Arkay's rise to godhood. Some sources claim he was but a man born of the earth, who ascended to godhood via Mara after learning the secrets of the cycle of life, death, and rebirth. Other sources state that he was present at the beginning, but he only found purpose after the creation of the world. I can't help but think of the rock-birth of Mithras in this case, with the 'maternal rock' being Nirn. After all, Mithras was a god AND a man. So there's that...
To bring this long-ass post to its conclusion (which isn't going to be short), I postulate that Trinimac's name is the key to the mystery: He is, as his name suggests, the "threefold son". He is the amalgamation of Arkay, Zenithar, and Stendarr into one being. This explains why he was considered to be the most powerful god ever to roam the world, and why he is connected to all three gods. We learn in ESO through a questline entitled "Mauloch and Z'en" that he has the power to overtake Z'en. As Mauloch's influence grows stronger in the world, Z'en's influence disappears. I believe the same case can be said of Orkey, with Mauloch overtaking Arkay's sphere and inverting it into a malevolent force. We've yet to see a similar instance with Stendarr, but it's well-known that Malacath abhors the weak and looks down upon mercy. That alone is an inversion of Stendarr's sphere. But that's not to say Malacath doesn't employ the influences of both Zenithar and Stendarr within his code the stronghold Orcs follow. Orcs are excellent craftsmen, and each weapon forged is done so in honor of Malacath. The code also employs 'righteous rule by might' through the chieftains of each stronghold. As for Arkay, well, I haven't figured out what his influence would be within the strongholds. Perhaps the Orcish views of old age and seeking glorious death could be connected to him. Oh, and here's a crazy thought: the weapon Scourge, Malacath's hammer, is said to be able to banish any of the Dark Kin, or Daedra, back into Oblivion. Not only that, but it has the ability to summon the souls of those banished enemies to aid the wielder in combat. What if this weapon employs the power of all three gods? Firstly, it's a hammer, and Zenithar's symbol is both a hammer and anvil. Secondly, it is potent against those who abhor the light, and Stendarr's light is used against those the Vigilants deem as abominations. Thirdly, it has power over souls, which is something Arkay is known for. Food for thought. But back to the main subject. How Trinimac managed to become a threefold god is a mystery. I suspect his removal of the Heart of Lorkhan had a part in it, since it played a crucial role in the formation and apotheosis of the threefold Talos. Perhaps the Heart's removal triggered a Dragon Break, assuming it wasn't already broken in the Dawn, and the Enantiomorph between Arkay, Zenithar, and Stendarr (the three cosmic order deities) resulted in the formation of a new god known as Trinimac, the ever-victorious warrior. But, unlike Talos, Trinimac did not have CHIM, and so his spirit was blackened and poisoned. I can't help but wonder if Mephala, Boethiah, and Azura attempted to become a threefold deity in murdering him. Put Arkay in place of Mephala (Thief), Zenithar in place of Boethiah (Warrior), and Stendarr in place of Azura (Mage) as the only who can tell them apart.
And there you have it, boys and girls. Oh, my brain hurts...