I kinda accepted that it is better not to think of the game's story or character's motivations. It's just a collections of stuff from D&D that Larian thought would be cool to used, stitched together.
I've begun to piece together the timeline of the last 100 years, and a lot of plot holes disappear if you down some wiki rabbit holes. There are still a lot of holes, but not as many as initially thought. For example:
The Dead Three have only been active as a team for the last 10 years, given Gorion's Ward probably died in 1482. They had to sacrifice some divinity to continue meddling on mortal affairs, demonstrating why they are so willing to exchange not having souls for an Illithid Empire in the Prime Material Plane. Myrkul only went to Ketheric 90 years after his defeat, as he was dead during that time, and basically used him as a puppet. As an extension of this idea, it is reasonable to see that both Orin and Gortash are more or less puppets of Bane and Bhaal, who don't really care about the interests of their chosen, but instead intend to use them to diminish the power of the other Gods by destroying/repurposing souls (depending on how much knowledge credit you want to give to the Dead Three) and to build up their own material capabilities by taking over the entirety of the Forgotten Realms. Ultimately, the antagonists serve the interests of their Masters, not their own, much as they think otherwise. You observe this when you try to redeem Ketheric.
The Elder Brain was in the vicinity of the Sword Coast for at least 450 years. It was likely the source of a large portion of Mind Flayer activity near Baldur's Gate for most, if not all, of the city's history. It was dominated only 8-10 years before the start of the game by the Crown of Karsus. This Elder Brain is likely very old, and had a Nautiloid hidden from its former days of glory. The Elder Brain would not have surfaced at all were it not for its being dominated. It is clear that it has been hiding for a long, long time. The Nautiloid is a rare planes-jumping device that was needed to steal the Astral Prism from Vlaakith. It was likely snatching people up as an emergency measure while being pursued by Githyanki to create more thralls on the go as cannon fodder. If you can create more soldiers to distract your pursuing enemy, why not. It couldn't go directly back to its home base, as it needed to lose the Githyanki pursuers, given a team of Gith with several Red Dragons would likely destroy the Elder Brain upon first contact.
Remember the human (This is a forum for a video game):
