After almost a day traveling in the the deeps of the mountains the Templars discovered the lair where that last soul gem was hidden:
Templars defeated the guardians of the lair and ventured back to
Krähenhort. Now, with all 4 Soul Gems in their possession, they would be
able to put the Banelord Valak Thul to final death. But that battle had yet to be fought...
game site note: Ogre Ninja Undead - even more awesome as it sounds.
That encounter was especially funny and entertaining. :) )