We know that they are individuals who failed in the Ascension process. Other than this, I have seen very little about them.
Did they become chimeras? Did they remain human? Did they gain power? Did they go insane? Were they outcast? Slaughtered? The lack of information regarding them in the Shuriman lore (as far as I've read) makes me believe that either the concept is underdeveloped as of now or that they have been purposely left out due to being some kind of taboo in the culture.
I've seen some assert that Xerath could be considered Baccai, but it seems to be widely agreed upon that he is just an unconventional Ascended.
Has there been an AMA or something that I've missed that has explained the concept a bit further or are we all left in the dark?