First, Hi, I am new to these forums but I have beaten torment.
Second, I know all the 3 endings, and the ways to achieve them I think..... without a walkthrough. The way to get all the endings is simple. Save in the mirror room before getting teleported out, so you can test all the endings to the talk tree.
The first possibility is to hand him his ass, either just willing him out of existance if you have 23 wisdom or higher, possibly using the blade of the immortal, not sure about any of the options with the blade, I have just heard about them, or just fighting him normally, with help from one of your ressurected party memebers or not. This way just shows a scene of The Transcendant One dying, and the the Nameless One getting sucked into and fighting in the blood war.
Second, you can merge with him, either by threatening to unmake yourself with will or threatening to kill yourself with the blade of the immortal, or forcing him with your will alone, The possibilities with using will to threaten him or force him require 23 wis, I think, so it's pretty much a mage only thing. If you do this, it will show you merging, then you will go ressurect and then talk with your party members, then you will get the blood war scene.
Third, you can REALLY unmake yourself with your will or kill yourself with the blade of the immortal, in which case it will only show The Transcendant one dying.
The chat tree can be a pain in the rear to work with, but be persistant and it should work. I am, like I said, not sure about the blade of the immortal 'cause I just haven't bothered with it before, I just heard about it from friends, so feel free to correct me if I am wrong. I am almost positive you can threaten to kill or kill yourself with it, its killing him with it that I am not so sure about. I am also not sure about the exact wisdom required for the will things, I have just heard it's 23.