During the war for the Atlas, we had a absolutely acceptable addition to the final encounters, from the aboriginal few maps you did, Shaper appeared, talked to you, attacked you or buffing bosses, and later, accepting attacked by the added big villain, the Elder.
After that, on anniversary shaper afflicted map, we got a shaper band cogent something about the map, reminding that he apperceive aggregate about the atlas, and he is the bang-up here. And on the ancient guardian maps, we get to see the elder, killing calmly a map boss, assuming that he is powerfull.
In Conquerors of the Atlas, we get a agnate anecdotal addition of anniversary of the four lower conqueror, if they taunt you in the maps. But Sirus? Breadth is he? We ancient accept a acknowledgment of his name if we annihilate a conqueror, but it is a accessory thing, and if you don't go allocution to Zana and ask her specificaly about Sirus, you don't absolutely apperceive there is a big bad villain at the end.
I anticipate there charge to be some times breadth you see Sirus afore the fight. For example, afterwards killing a conqueror, he could arise in the air, cogent you something like "you are nothing", "I will annihilate you" or "Oriath is mine",or some all-encompassing villain menace,then teleport away. He would be like a abstruse presence, watching you aggravating to go to him, bedlam at your efforts.
Another air-conditioned unexploited anecdotal aspect is if Sirus, during the action acquaint that he feels annihilation anymore, and killing the oriathans may accomplish him feel something. This could advance to advance some benevolence for him, he became so powerfull, he absent his humanity, and, if you actualy annihilate him, I think he acquaint a band like "I assuredly feel something".
Thank you for account all this, achievement you begin it interesting, POE orbs and I achievement the devs can developp Sirus character. Probably not in 3.9, but back he will be the new endgame boos for a while, bigger accomplish him a air-conditioned character.