Tethtoril is a human located in Candlekeep, in the garden in the inner keep.
Like Gorion, if you try to attack Tethtoril, he will immediately cast "Call Lightning" spell, which instantly kills you.
Canonically he is a Level 16th Priest of Mystra. In the game he is a level 18 with no class.
|“||Firebead has sent you to me, hasn't he? Very well, return this scroll to him, but then you must hurry and speak with Gorion. He is waiting for you on the steps of the central library. I assure you, child, it is a matter of greatest urgency.||”|
–-Tethtoril, if you have already spoken with Firebead Elvenhair