I'm pretty sure that what we beat was *A* siren, not *THE* Siren. Though it's also possible that it was indeed the true Siren, in which case it's perfectly possible for her to return for later content much like Diabolos and Ferdiad did.

On the topic of underwater dungeons from earlier in the thread...maybe we'll get a Quarn HardER mode in the expansion? I can't be the only one who read 'Sunken Temple' and got all excited, only to be viciously let down when it wasn't a gorgeous underwater dungeon.