Sure, Sion seems to be the biggest monster there is in the show...
And why stab Ryner now?
We can assume it isn't a fatal blow, but I wonder what purpose it will bear to the story.
Also, the red hair guy getting a fully functionnal arm is a little too convenient compared to what we've seen up to now in the show. I really hope he does not have his power anymore, or that would just be bending the facts the story has painfully set till now.
Anyone could always argue that since that world is so far away from ours, we just never got the chance to see such a marvelous prothesis up till now, but I feel it was still cheating. Might be an arm from a dead body, instilled with magic, I know it the most logical answer that fits with the story values.
And yes, the story developments might be steep from now, since they never really have been fast up till now, and they continue to add more questions, when there are only 5 eps to prepare a conclusion... unless they plan on finishing it in OVAs?