King Arthur did not die just yet. As an immortal, his tenacity of living still tried to get the best out of being nullified. Of course, his end is still the same as he does in the real world. The only difference is that Bedivere is not there to witness his eventual death.
"I don't know what is going on, but how did someone like you managed to even kill me...it doesn't make sense..."
"It's Mordred's descendant who killed you, not me. I am just a catalyst for your eventual death but that's all I have in my job here. In fact, this event is just for us to receive your relics and nothing else. Perhaps the main reason for you still existing is for the usurper to eventually defeat you. You might be immortal but you never have been a dangerous guy with all the things I did to ensure the eventual victory."