Okay. Someone help me with this. As far as I know even Blizzard hasn't been entirely clear, nor has the Arthas: Rise of the Lich King book.
We all know the "original" Lich King was Ner'zhul and then his spirit was joined by Arthas' at the end of The Frozen Throne, apparently becoming "one" entity (remember "Now, WE are one!" in the last TFT cinematic?). At first, before any retcons, they were "fused" and became one entity, IIRC.
Fast forward to the original level 60 Naxxramas, it appears that the voice of the Lich King when you approached Kel'thuzad sounded "double", as if both Arthas and Ner'zhul were talking at the same time, saying exactly the same words. This implies that, at least by this time (as in, before Arthas took on the lead and killed Ner'zhul's spirit, or maybe before Blizzard made this change with the book! -retconned- cough.), the Lich King was going to have a double personality or perhaps he was going to be like the ogre race april's fools from Blizzard, with problems controlling the body, heh.
Then, after the book, it appears Arthas has taken the lead, and destroyed Ner'zhul's spirit. According to quests in Icecrown, the Lich King is one entity that is no longer Arthas nor Ner'zhul anymore. Just the Lich King. Then it is said that Arthas was holding the Lich King back? How is this possible?
Now I'm lost. Was the LK at one point a double person entity? Was the LK always the LK, no Arthas, no Ner'zhul? Was it Arthas the LK, but unconsciously holding back the Scourge? Or was the LK one entity, where deep down Arthas still struggled, holding back the Scourge from within? Was the LK ever Arthas at some point? So many questions here. Hopefully someone can help and clear this out for me.