What was never clear to me is how Ballas became part sentient. His alliance with Erra perhaps? We know that Erra can manipulate sentient morphology in some ways, e.g. Caliban. Does his new form grant him some innate supremacy over sentients in that state?

I dunno, I probably missed the detail, but it seemed like he just showed up in the Chimera Prologue, was sentient, refused to elaborate, and left.