There is no key for that door I think (hope I'm right):

I think
that door can only open when Sassan comes with you for going to the Soul Forged Demon of Laiken... But first you must find his "name" in a book ...

On 7th of february 2015 : I start a new adventure in the Divinity world of Original Sin,
it's a Fantastic Freaking Fabulous Funny ... it's my All Time Favorite One !