Yes, Arya stabbed him with Valryan steel (even if she ran five miles in 15 minutes through a load of the dead - THEN was still able to sneak up on The Night King and another hoarde of dead/NK lieutenants and stab him to death whilst being choked)...
But could he return in some spectral or warg form, etc?
He still could be the personification of Winter.
After all, the CotF created him in a place that later was north of the wall, and there wasn’t the slightest sign of an arctic climate at that point. So maybe there was no winter at all before he was created.
Winter had to exist before the NK came to be, where else could the CotF get the idea? And how could he be the personification of forces that did not predate him?
Nah, he was probably just created during a summer, when even the far north was green. Even Alaska and Siberia have a green summer, and BTW the CotF probably found the humans to be at their most bothersome during summer.
While it’s right that there’s not a lot to support my "theory" so probably it IS wrong…
> Winter had to exist before the NK came to be, where else could the CotF get the idea?
Obviously, there were things about their creation that the CotF didn’t expect. For example, killing them too instead of just killing humans. They maybe didn’t fully understand the magic they were using.
So winter may have been another unexpected side effect.
(Of course, there might be other reasons why creating the WW went so wrong for the CotF, and it’s a shame that the show didn’t explore on that.)
> And how could he be the personification of forces that did not predate him?
Worked well for the Endless in Neil Gaiman’s "Sandman" comic book.
> the CotF probably found the humans to be at their most bothersome during summer
They just made him essentially into an evil cliche character that wants to kill everyone. He even gives that smirk after the dragon's fire doesn't work on him. So much opportunity to make him more meaningful (and powerful) and they wasted it.