Ceresa was killed by some monsters (yes, they do attack the ai; one city was levelled by a troll while another by a group of ophidians). However, one of her mirror elementals was left behind on the strategic map. It wouldn't move, but I could talk to it, leading to a rather awkward conversation with myself in an empty throne room.

The elemental was standing on a water shard, which makes me suspect it spawned on the exact same turn that Ceresa was eliminated, and a race condition resulted in the elemental not being removed from the world.