Originally Posted by
TemporaryVideos
The moment Arthas put on the helmet, in the end of the Frozen Throne campaign, both he and Ner'zhul ceased to exist as individual beeings. Their spirits, minds and memories have merged into a single entity henceforth known as the Lich King.
What we're dealing with in the RotLK novel is a conflict between the last remnants of their singular personalities (think of schizophrenia), NOT the beeings themselves (as they no longer exist at that point), NOT their souls (as Arthas' soul is not even a part of the Lich King). Now, Arthas' personality emerged victorious, which, in addition to his former body beeing the host of LK's power, is why he is beeing equated to Arthas throughout the WotLK expansion. Make no mistake, we've fought neither Arthas nor Ner'zhul in ICC. We've fought the Lich King, an entirely separate beeing.
Why do you think replacing the Lich King was considered such a great sacrifice on Bolvar's part? It's because it wasn't about taking its powers, but becoming its next host. It was about taking on a burden of eternal struggle with the will of the previous masters of the Scourge. Exactly as in Arthas' case, Bolvar as we knew him is no more. His entire mind, memories, skills and personality are now part of the Lich King, merged with those of Arthas and Ner'zhul.
There should be no surprise in his voice beeing identical to the one of the the previous Lich King. Because it actually is the very same Lich King, just with another layer of personality added on top of its former self and with a fancy (?) new host body.
Arthas haven't killed Ner'zhul. The Lich King did. And, as I've mentioned above, it wasn't Ner'zhul per se, but only his personality.
The Lich King has the memories of both Arthas and Ner'zhul (and now Bolvar, as well), so it's natural for him to remember the time spent as a shaman.
Their personalities weren't simply overwhelmed, they were fused, they are what makes the Lich King. They are gone and yet still there, just not separate anymore.
Arthas (his soul) was never part of the Lich King, as it was absorbed by Frostmourne. His mind/personality was. And mind/personality does not die, it's still there, it's part of what the Lich King is.
This is directly contradicted by Blizzard's later statements and materials. Blizzard changed direction they were going with that. Initially they were a perfect fusion, yes. But they later decided to keep their spirits separate. Arthas destroyed and consumed Ner'zhul's spirit before he awoke on the Frozen Throne. Arthas is intended to be the "ultimate expression" of the Lich King.
Arthas murders the manifestation of his humanity, then destroys Ner'zhul:
“We are one, Arthas. Together, we are the Lich King. No more Ner’zhul, no more Arthas—only this one glorious being. With my knowledge, we can—”
His eyes bulged as the sword impaled him.
Arthas stepped forward, plunging the glittering, hungering Frostmourne ever deeper into the dream-being that had once been Ner’zhul, then the Lich King, and was soon to be nothing, nothing at all. He slipped his other arm around the body, pressing his lips so close to the green ear that the gesture was almost intimate, as intimate as the act of taking a life always was and always would be.
“No,” Arthas whispered. “No we. No one tells me what to do. I’ve got everything I need from you—now the power is mine and mine alone. Now there is only I. I am the Lich King. And I am ready.”
The orc shuddered in his arms, stunned by the betrayal, and vanished.
...
Arthas, the Lich King, alone in his glory and power, slowly opened his eyes.
--Rise of the Lich King
In the manga Legends: Fate, Arthas says, "Ner'zhul is no more. He is consumed. There is only Arthas now..."
During BlizzCon 2010, Metzen said Arthas was the ultimate expression of the LK and that Ner'zhul is finished:
Originally Posted by Cris Metzen
Q. I have a question regarding the storyline of Ner'zhul: is he finished, or what's going on with that?
A. "Yes. Yeah, Ner'zhul is done. He served us well in that capacity, but really the Lich King idea... really has the ultimate expression, you know, as Arthas or whatever. Ner'zhul's done." (
Source)
The Ask CDev says Ner'zhul and Arthas had separate reigns as Lich King:
Originally Posted by Blizzard Entertainment
Yes, that does also mean that Arthas and Ner'zhul were not unleashing the full force of the Scourge during their respective reigns: you are welcome to speculate on the reasons for that. (
AskCDev2)
Metzen says Ner'zhul is completely gone:
Originally Posted by Chris Metzen
Is ne'zhul completely gone gone?
Far as I'm concerned.
(
ChrisMetzen)
All of the above items came after WotLK was released.