It's random and for you to fight him you first need to find a way to get in, your character only starts looking for a way in after you take a task from the day witch to get her a lizardman egg. After getting in you have to roam around a little. But you can only fight him after you unlock the purification lake.
To unlock the purification lake you need to gather the three piece of the broken lizard sculpture the head is the first you find in the swamp before coming to the temple, the torso you get via a task from the swamp witch and the tail you find in the bandits camp, when you get a piece go to the inner shrine of the temple walk to the end of the corridor and of you have a piece of the sculpture in your inventory there will be a option to put it in place.
Do you have the corrupted lake as well? Is in the underground of the forgotten temple if not try going to the witch (try going in the daytime and nighttime not sure when it appears) to get the task of the egg I mentioned, if you do have the corrupted lake then Ohtra should have a dialogue option with a yellow '?' Talk with her and you'll mention saving Roushk after that you go to the corrupted lake and wait to ambush him.