I'm currently on a very big map, very slow tech, all victory conditions off (those that can be off) & hundreds of turns in when suddenly one of the NPC empires (Magnar) declared that he leaves the world and makes it look like his empire was never here. Which he did. Lots of empty plots in the far east where his citys where located from one moment to another + the obligatory treaty expired messages (+ one "1 unit deserted" message).

Is that common? Is that a special Magnar Diva feat?

IMHO that is one "human skill" the AI should totally not try to emulate in any game for added realism; if thats what it was.