Thul Za Dhuum's name is possibly a reference to the Robert E. Howard character Thulsa Doom originally from a story set in his "Kull" universe, but much better known in his incarnation as the villain of the movie "Conan the Barbarian".
He will use Signet of Humility when your elite skill is recharged, even through Shadow Form, so pay attention that you recast it as soon as possible.
