In a novel side story? Huh. I have all 19 mainline volumes and dont remember that. Granted, there is a LOT to remember from 19 volumes, let alone all the spin offs, let alone one from so early in the series. So I probably just forgot.Fels reattached it. It happened in a novel SS so you'll have to read that. Basically after Ais severed his arm the other Xenos picked it up and froze it to stop decomposing. They knew Fels would be able to do something about that.