I've been visiting Nizara a lot trying to fill in old collections, and I have killed hundreds of the lamia that inhabit the area. There's not a lamia slayer achievement. These aren't naga, like the ones in Pillars of Flame, because they have arms.
I'm not an EQ2 loremaster by any stretch, so maybe I'm wrong, but aren't Nayad and Lamia different races? They certainly look very different.