Is there a quest involving the prisoner in the Serpent Temple? I can't get him to talk to me. I've since moved on and just finished the Castle. Should I go back?

His name is h'Thark. If you never got the quest from Gorthius to give him his lockpicks, you can get him to tell you more about the Snake Temple quest (getting the Masque of Death) by "using" the iron rod on him to make him talk.
Seeing as you have finished the Temple, you really have no need for him unless you are looking for someone to torture!

; )
Moni

Did you complete the Serpent Temple? Destroyed the Naga, got the mask? If you've done that there is no need to go back.

Wyv

After I forced him to talk, out of guilt I healed him.

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chance favors a prepared mind

Thank you. I must have missed Gorthios. Where is he?

Head southwards outside the East gate of Valeia

OK, thank you.