Arrigus Mistborn is a human wizard of High Sorcery that wears black robes. His familiar is an owl named Nightshade. During the Test of High Sorcery, when given a choice, Arrigus chose power - but it's unclear - even to those who observed - what had exactly transpired, because the entire room had gone completely dark as if what was being said was not meant to be seen or heard by anyone else. After the darkness lifted, Arrigus collapsed to the ground - the irises of his eyes gone - replaced by black circles as if Nuitari lived in his eyes. He revealed to adventurers, that he'd made a pact with Nuitari that was made in secret - and that he was working with Nuitari to ensure Takhisis does not win the war; though Nuitari was the son of Takhisis, he believed should she win, she will demand all of his magic go to her and so Nuitari has hand picked selected wizards of the Black Robes, and promised them power, should they help ensure that his mother does not win this war that is coming. Arrigus has mentioned an interest in saving an Irda; it's unclear how he knows of this Irda, but it can be speculated that it's knowledge given to him by Nuitari.