I think Kel’Thuzad was this character, but Blizzard did a very poor job presenting it. However, seeing Ner’zhul’s soul in Torghast reminded me of a scene for Kel’Thuzad that Blizzard rarely touched upon, being Kel’Thuzad meeting Ner’zhul:
Fight or flight: those would have been the heroic choices. Heroic, but pointless. His death would serve nothing. By agreeing to become the necromancer’s apprentice, Kel’Thuzad bought himself time in which to bolster his own skills. With enough training, he could surpass the necromancer or catch the man off guard.
You will never catch me unaware, for I do not sleep, and as you should have already guessed, I can read your thoughts as easily as you might read a book. Nor can you hope to defeat me. Your puny mind is incapable of handling the energies I manipulate on a whim.
Kel’Thuzad had long since torn his robes, and his leggings were useless against the icy rock of the rough-hewn stairs. His hands and knees left bloody tracks behind him as he struggled up the last spiral. The throne radiated bone-chilling cold, and mist surrounded it. A throne not of crystal, but of ice.
Immortality can be a great boon. It can also be agony the likes of which you have not yet begun to fathom. Defy me, and I will teach you what I have learned of pain. You will beg for death.
He came within a few feet of the throne and could go no farther, pinned helplessly beneath the thing’s overwhelming aura of inhuman might and hatred. An unseen force bore down on him and ground the side of his face into the unyielding stone. “Please,” he found himself sobbing. “Please!” Further words escaped him.
Finally the pressure eased. The wraiths flitted away, but he knew better than to rise. Doubted, in any case, that he could. His eyes, however, unwillingly sought out his tormentor.
A set of plate armor was seated within the throne, rather than upon it. Kel’Thuzad might have thought the armor merely black, but, blinking hard, he saw that no light at all was reflected from its surface. In fact, the longer he looked, the more it seemed to devour all light, hope, and sanity.
The ornate spiked helm obviously doubled as a crown. It was set with a single blue gem and, like the rest of the armor, appeared empty. In one gauntlet, the figure clasped a massive sword whose blade had been etched with runes. Here was power. Here was despair.
As my lieutenant, you will gain knowledge and magic to surpass your most ambitious dreams. But in return, living or dead, you will serve me for the rest of your days. If you betray me, I shall make you into one of my mindless ones, and you will serve me still.
Serving this spectral being–this Lich King, as Kel’Thuzad was beginning to think of him–would assuredly bring Kel’Thuzad great power… and damn him for all eternity. But that knowledge came far too late. Besides, damnation had little meaning without the prospect of true death.
What I could see is the Jailer offering Kel’Thuzad revenge against Ner’zhul, with the power to surpass and torment him, as we see the state of Ner’zhul in the raid. With Kel’Thuzad never actually having been loyal to Ner’zhul, and having expected Arthas to replace Ner’zhul, only for Arthas to fail to meet the Jailer’s expectations as well, Kel’Thuzad might have just been playing along all that time, waiting for the moment to appease the Jailer, as he finally admits to us he’s been doing all along when we come to fight him one last time.
