duma

As far I remember, Sigzil said clearly he was a member of two different orders. It could be anything (he didn't say specifically Radiant Order) but also a reference about Aux not being an honorspren. But I thing the main clue is the illustration included in the book, when Aux burn itself to give Sigzil a full radiant power, the symbol represented in the fire (and described by some other character) is from Skybreakers. So I guess he bounded Aux (highspren) after he ends with Windrunners. We also know that he lead his former order to some disaster in Alezkar so maybe he left them after some kind of trauma. Aux death is caused when Sigzil overused the dawnshard power as he described in the book. No relationship with oaths breaking. If his former bounding with an honorspren caused the dead of that spren has not been explained (and it is strange for me because I would have expected Sigzil to be guilty about that).