I'm most of the way through the Serpent Temple and now I need the Heretic Key. Only problem is, I can't find Kol. I've spent a lot of time in front of the Temple looking for him: hanging around the entrance and looking through the woods but he never shows up.

Before I entered the Serpent Temple for the first time, I saw Kol near the front entrance, but never spoke to him, and I had to go back to Ishad N'Ha for something. I've never seen him since.

Does Kol go somewhere else or do I have to wait a LONG time before he shows up?

same thing happened to me....I had to go back to a prevously saved game

i dont know why you need to find him...im sure i will need to soon enough then..but i can't find Kol either ...like you said he was there the first time i came to that castle and then after that ...he was never seen again...not here not there not anywhere haha..hes gone...and i dont have any saved game from befor that...i save often ...over 150 times im sure..do i need to find him later on in the game???

Kol has the heretic key which of course you will need in the Serpent Temple.
I've read of other people only half talking to him before going off and doing something else in which case Kol seems to disappear. As far as I know he isn't going to reappear near the Serpent temple though he is to be seen later in the game. This is a serious problem which needs to be addressed - the game needs a patch.

I can try to walk you through a workaround if you are familiar with hexed and I can find the heretic key in my own party's inventory. Not the best solution but if you are really really really stuck and want to give it a try, email me.

Wyv