Just a few housekeeping notes, and this is no reflection on Gfunk. Some of this info he was not privy too, since he is not DM'ing the adventure, so I'm just going to clarify a few points.
Nabthatoron was not imprisoned in the abandoned temple where Alek Tercival was found. He was bound to the PRIME due to his defeat at the hands of Surabar Spellmason. He was actually residing in the Demonskar, and had been scrying on Alek. Alek had originally stumbled across Vaprak's Voice, the home of the hags, not the desert temple. There he fell under the hags Domination ability because he had drank the Amaranth elixir, making him stronger, but sapping his willpower (ie...+8 strength, -4 wisdom...for the record, Rusty drank the same thing prior to the hag fight). He became a pawn of the hags, who gifted him to Nabthatoron. The demon saw his chance to gain revenge of the descendents of Spellmason by having Alek go to Cauldron and issue the challenge there, thus instigating the riot and the declaration of war by Cauldron on the Chisel. Nabthatoron planned for the army of Cauldron to destroy Red Gorge, thus ending his exile.
All along, Alek believed the hags were truly archons, and he trusted them, in his diminished state. When he returned to them after making his challenge, they instructed him to enter the Starry Mirror. They told him he would see prophecies of the future there. However, once he entered the mirror, he found himself in the desert temple, and was trapped there. He had been there for weeks when the Bright Axes found him, and was insane with despair at his predicament, and his betrayal by the 'celestials.' When the party found Alek, Nabthatoron had to act to prevent them from freeing him and having him retract his challenge, thus sparing Red Gorge. Causing Alek's death was the most direct means of doing this, though he would have preferred to prolong the paladin's torment.
Hope this clarifies a few things.