Tethtoril is a human located in Candlekeep, in the garden in the inner keep. If you speak to him before speaking to Firebead Elvenhair, he will tell you to go and see Gorion, then he will walk away. 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 Cleric of Mystra and one of her Chosen. In the game he is a level 18 with no class.