Slay Mragesh and return to Zurjaya in Zabra'jin.
You've dealt with most of the little hydra, but I doubt Mragesh was impressed by your display. He's a cunning, ferocious beast and he's dogged many an angler in his time.
I intend to be the last one he'll ever bother. He's bound to be hiding deep in the waters of Serpent Lake, to the northeast. He's a has-been; he just doesn't know it yet.