[EDIT (though I doubt anyone will read this): To all the people asking, Astarion does not die in this ending. The tadpole is gone because you destroy the netherbrain, and so he's once again vulnerable to sunlight. He just runs off to hide himself. It happens in EVERY ending where you destroy the netherbrain, and has nothing to do with you becoming a mindflayer or siding with the Emperor. The only way to avoid it is to let Astarion become the vampire ascendant, which can be done as part of his personal quest.]
I couldn't find any video showing this ending, nor any information on how to get it. I only found a video in which the player has their mindflayer!Tav choose the option to eat the Emperor's brain...which, fair, I guess, but I wanted to see the version where we ride off into the sunset with our morally questionable octopus prince! So I decided to be the change I want to see in the world and recorded the scene myself for all my kindred spirits.
I first didn't get this ending, even though I'd embraced ceremorphosis. The Emperor effed off with nary a word. I read a post saying you must have no recruits at all, but then I reloaded the save and just chose different dialogue options. I had chosen ''I like what I've become'', but then ''A quick rest, then--to work. There's a lot of good a mindflayer can do (...)''. Turns out, the second option: ''I'll do what I like.'' is the correct one if you want the Emperor to offer to stay! And as you can see, all our homies are still there.
Enjoy!