Can anyone tell me where the 3rd level spell "invisibility" can be found? I found it once, the first time i played. I seem to remember it being in the final crypt in the Vale of shadows, right near the entrance to Kresselack's chambers. Is it a randomly appearing spell??

Its a second level spell, but sorry cant remember where I found it. Like you said may be a random one

You can purchase this spell and it does randomly appear.