There are supposed to be additional mage spells you're able to cast, but I've been unable to find these spells, even on completion of Tales:

Improved Invisibility, Polymorph Other, Spirit Armour, Shadow Door

Does anyone know where to get these spells?

PS The final fight with the Tanar'ri was one of the most challenging I've encountered!

You can buy Improved Invisibility in the tabern-shop of Ulgoth´s Beard, but the other spells are only available cheating . Another spell I haven´t found in the game is 'Cone of Cold'. I found it hex-editing.

Thanks man! A pity that I can only get them via hacking though