Out Of The Abyss
Prince Derendil looks like every other quaggoth you have seen while you were in Velkynvelve: Hulking, dangerous, and with razor-sharp claws. However, when Ara spoke to him, he responded in urbane, refined Elvish.
The prince told you that he is not a quaggoth at all, but instead, he is a gold elf. He is Prince Derendil of the kingdom of Nelrindenvane in the High Forest. He was cursed by an evil wizard named Terrestor, who polymorphed him into this form, usurped his crown, and exiled him from his people.
Derendil is prone to go into bouts of rage in stressful situations. When it passes, he is remorseful, and has confided in you that he fears he is losing himself to the savagery of this quaggoth form.
As Ara was the first person to approach him, and he knows she speaks Elvish, he tends to hang around her.