The Transcendent One was created after The Nameless One was rendered immortal, a living embodiment of the concept of that man's mortality. He is glimpsed throughout Planescape: Torment, erasing the trail The Nameless One takes, murdering people who were important to The Nameless One finding out the truth behind his immortal state. This is because he seeks a singular purpose: he wishes to remain a separate entity from The Nameless One, and he has decided that the best way to do so is to make it impossible for The Nameless One to discover the truth behind his condition, and then to murder him in such a way that he once again loses his memory and starts a new incarnation, this time with no hope of ever regaining his mortality, and thus ending The Transcendent One's existence.
In order to better facilitate his victory, The Transcendent One will attempt to sway certain members of The Nameless One's party against him. For example, if Vhailor is in the party, and The Nameless One's alignment is Evil, The Transcendent One will turn the justice-obsessed Mercykiller ghost against his friend by informing him of some of the many, many crimes that The Nameless One's various incarnations have committed over the centuries. Similarly, if Ignus is in the party, and The Nameless One has a Good alignment, The Transcendent One will turn him against The Nameless One... well, fairly easily, actually, Ignus is totally insane and will take literally any opportunity to use his magic to burn things.
The Transcendent One was voiced by Tony Jay.