I learnt he is hiding in the forest and hidden by magic. I should use my senses. Well I guess i need the anti - Illusion potion. Ok but where? Used it on the ruins but then no idea.

Ah Found the spot. Do search properly ;-)

Since it sounds like you want an explicit answer, it's in a clearing. You can find it from the first forest room by going NNNWNNES

And you can't see it properly without help, which you already have by the time finding Brashem becomes an issue.