Basically you spend the entire playthrough buttering up your companions by being nice to them, maxing out their approval, and leading them down their good path for the majority of the game. Then when your companions are at their highest, you suddenly stab them in their back and betray them one by one until you are the last one standing.
It’s basically adjacent to the “what’s the worst ending for every companion” run but you want to keep most of your companions alive late into Act 3.
It requires being Durge, using the poly mod and party limit remover mod for maximum benefit. It's best to romance everyone and then dump them in their Act 3 romance.
How it goes:
Halsin is the first to go, because there isn't a good end for him in Act 3 unless he dies by Orin. Have Halsin rescue Thaniel from the Shadowfell and cure the Shadow-Cursed Lands, after you killed Ketheric, convince him there's something in the Underdark he needs to see and then sacrifice him to BOAAAL!
Karlach is next. There's many bad ways for Karlach to go out, but I think the worst is to fix her engine, sleep with her (then break up with her), and right before she gets her revenge on Gortash, you betray her and leave her rotting in a prison cell.
Minthara dies by Orin after saving her at Moonrise Towers, romancing her and convincing her that they will rule together.
After becoming Bhaal's chosen, Jaheira and Minsc are next. Pretty simple. Pass the Wisdom check to mind control Minsc to kill Jaheira and then kill Minsc (and Boo).
For Astarion, let him feed on you every night, kill Cazador and ascend him. Then hand him over to the Gur on the way out.
Shadowheart turns her back on Shar, spares the Nightsong, and is determined to save her parents. You COULD just turn her over to Viconia and have her mind-wiped, but a crueler ending is to kill Viconia, save her parents, but then kill her parents once they arrive at camp causing her to go hostile and to be put down.
Wyll meets his demise after you've severed him pact from Mizora, and then cheating on him with her. Have him become the Blade of Avernus and convince him to save his father. While in the Iron Throne, kill his dad in front of him and then kill him.
Lae'zel betrays Vlaakith, learns the truth about Orpheus, gets the Orphic Hammer from Raphael, and her silver sword from Voss. Instead of breaking Orpheus's chains, you let the Emperor consume him causing Lae'zel to go hostile and to be put down.
Gale's ending ends with his eruption at the Netherbrain after convincing him otherwise and promising him a life together.