Arcane Essence
Ten years is, apparently, long enough to forget about a less-known race when it disappears, replaced by a new kind of monster that shares a few of its traits.
Of course, that's kind of inevitable. Mobs don't just appear out of nowhere - well, okay, they do, but not like that. New types of mob don't appear out of nowhere, and that happening kind of threw the world out of balance for a while as it struggled to adjust. A race vanishing, when it was already rather secluded and rarely-seen, would be a bit overlooked.
Especially when the new mobs seem specifically designed so you won't notice the fishy things about them. *Taps forehead* you can't see the pain in a creature's eyes if you can't look it in the eyes!