Arthas effectively ate Ner’zhul. There are still “vestiges or at least memories” of Ner’zhul within the helm, but he’s dead and gone.
Arthas died, and since he was not bound to the armor like Ner’zhul, he is also mostly gone. Some memories are clearly leftover, but that is likely the extent of whatever remnants of the two remain.
Relevant citations.
Arthas: Rise of the Lich King, epilogue
“Arthas, the Lich King, alone in his glory and power, slowly opened his eyes.”
Warcraft: Legends Volume 4 - Fate
“Ner’zhul is no more. He is consumed. There is only Arthas now… Arthas, whom you shall serve for all eternity.”
Then there’s also the quotes Dreadmoore has given you.
and finally on an unrelated note
Mauden#1892
Fair warning though, I’m the exact opposite of talkative.