I'm still chewing over the Alien intervention. The writers must be up on the current speculations or have watched Alien series on the Discovery Channel. Supposedly they brought knowledge with them to Egypt and Middle America so the writers tapped into that as at the "later" age where Merlin is set (and on a different continent) it could be that a group was chased there...not likely...and killed off so that only one remains, eternal.
That it said Arthur's Bane is himself will play out now that he has (though an act of benevolence) knighted Mordred - the Druid prophesied to kill him. I wonder, though, if Merlin will seek the Alien out again for other knowledge?