In addition to Unique items that can only drop from Grigoire, you also have a chance to receive the Demonbinder Mount Armor Cosmetic.
The Flesh-Weld Rod Trophy has a chance to drop from this boss in addition to Unique items that can only drop from Varshan.
The Skull Torch and Fell Steed Mount Trophies can potentially drop from this boss, in addition to Unique items exclusive to it.
The Diadem of the Ancient Helm Cosmetic can potentially drop from this boss, in addition to Unique items exclusive to it.
Out of this group of Bosses, Duriel is the only one who has a chance to drop Uber Unique items. The smoldering Brimstone Mount can also potentially drop from this boss.