Hickory: The Nameless One was an immortal who lived many varied lives, spanning thousands of years, so your question doesn't make much sense. He was made immortal in a flawed ritual that made him lose all memory each time he died. He eventually confronts his 'mortality', embodied as The Transcendent One, and has to make a choice. Whichever choice is made, he becomes mortal again, and ends up walking towards a battle in the blood war between demons and devils.

Shaolin_sKunk: Possible spoilers! In the fucking title. Nice job.

DarrkPhoenix: The assassins were shadows sent by the Transcendent One. The goal was not to actually kill TNO, but through death to make him forget so that his present incarnation could never figure things out to enough of an extent to make it to the Fortress of Regrets and come into contact with TTO. In his original life TNO became condemned/contracted to fight in the blood war (we never learn the exact details of how this came to be), but to get around it TNO put together the plan to separate himself from his mortality. Ravel helped TNO carry out this plan, which successfully removed his mortality in the form of TTO. However, as an unexpected side effect TNO would at times lose his memory upon dying. On the other hand, TTO still remembered everything about TNO's life, and maintained the goal of keeping them out of the blood war by keeping TNO and his mortality (TTO) separated. Thus why he sends shadows to kill TNO, to reduce the possibility of TNO finding a way to regain his mortality by basically resetting his memory (through death) on a routine basis.

Jonesy89: Making it even worse is that the Shadows are in fact the spirits of people who have died because of TNO's immortality. Whenever he "dies", his missing mortality results in him rising while someone else on the Planes dies in his place, so those Shadows have a massive hate-on for him.

As to the Blood War stuff, the game actually doesn't touch on this (or at the very least, if it does, then I do not recall it). I actually remember one adaptation of the game into a novel around the time of release having Fjhull Forked Tongue stating exactly what TNO did that caused him to become damned, as opposed to leaving it purposefully unstated; personally, I like the latter approach better, as trying to define whatever TNO did would always diminish it and draw attention away from the true focus of the story (the impact it had on the main characters and the Planes as a whole), in much the same way that telling the audience what was in the briefcase would have made Pulp Fiction nowhere near as enjoyable or memorable.