That's what I have been pondering about for a bit. I can't say much about Durge, because I lost interest in that very quickly, but for the neutral/good Origins who end up with Ascended Astarion and then make a grab for the ultimate power, it is very hard to not read Astarion as at least part of the driving force behind this shift.
I can only say it with certainty for Avatar-Gale, in who's case Astarion is written as the devil on your shoulder, who encourages you to consider your own will over Mystra's, who's word about controlling the artefact at the heart of the cult run through your head before going to sleep and who praises you oh so prettily for every power-grab you make. It's a shame that they simply had the romance drop for Avatar-God-Gale (especially if said god is also a vampire lord's spawn) but not having him (romanced Ascended Astarion that is) feature prominently in the Absolute ending, would be giving up the subplot that started in act 2 and could now be realised. (Maybe even with some back-stabby action by Astarion if you didn't control him but also didn't become his spawn akin to the House of Hope ending, or some allusion to him planning to puppet-master the newly minted Absolute.)
Thanks for actually bringing that up Anska. That's why having these discussions is important.
Until you mentioned it yourself it completely slipped my mind (due to some of these unique Origin ending issues) that Minthara/Astarion as love interests are actually supposed to be present in
every other character's Origin ending too (if they were romanced). And because of that every ending has to properly account for having them as their love interest, especially if the avatar Origin ends up becoming a vampire consort under Ascended Astarion.
But after seeing how the love interest was treated in Chosen Dark Urge's ending, I'm now even drastically more concerned how they're portrayed in other character's Origin endings. What does the love interest do if Lae'zel becomes the Absolute? What do they do if Gale becomes the Absolute? I cannot even begin to imagine Wyll or Karlach's love interest in their ending. Does the game just miserably push Ascended Astarion to the side so the Origin character can have their evil moment? Or does he interfere and say;
"No, we're doing this my way precious." I truly am curious how the romance aspect of each unique Origin ending is treated, because Minthara and Astarion are supposed to be by every Origin character's side till the very end. Had Larian spent more time and effort into these endings I would've loved if there was an actual power dynamic going on between Ascended Astarion and an avatar Origin becoming the Absolute. That would be so fun!
Because the Emperor can fully dominate the player, so I would hope Ascended Astarion can too. Would be very interesting roleplay, imagine how awesome it would be if the player romanced Ascended Astarion till the very end with a Chosen Dark Urge and then Astarion took control of him, ultimately using him as a sanguine weapon to butcher the world but deny him from fully fulfilling his purpose.
Now something like that would be awesome
