Tek'elach is a fiend that the party encounters in the northeastern section of Curst Underground. He is a greater baatezu of the impressive rank of Cornugon. He is cooperative and answers questions, but makes it clear that he is evil. He is very elusive regarding his presence in Curst Underground and will not elucidate on that piece of information. He will mention that the high levels of Chaos in Curst are causing Curst to shift alignment and "slide" into the plane of Carceri. Pay attention to what he says about the balance between Chaos and Law and how a Chaotic act may be counterbalanced by a Lawful act. This knowledge will be invaluable later in Curst in Carceri. Speaking of which, look for Tek'elach and Agril-Shanak battling by the Barracks in the aforementioned Curst in Carceri.
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If TNO is a Godsman, be sure to mention TNO's knowledge of the weapon the Godsmen are crafting for the Blood War. You'll have to press him, but what he has to say is rather enlightening. (Not true? On my savegame and three people's playthroughs on Youtube - one of them not on the EE - he only mentions that it is used in the Blood War and that it was commissioned by "his superior", and there is no way to press him further. Is he encountered again after Curst Underground?)