I must say, Rei was rather dumb in this chapter; sure, he was smart enought to catch the vial and throw it back at Well, but when We'll is severely wounded missing an arm, why just stand there and watch him, even egging him on to pull out his next trick, why just not kill him before he has the chance to do something more?? 
The fact that Well escaped is 100% Rei's fault, he could have stopped him, but he just stood there and let it happen. And he can't complain, even a little bit, when the Empire learns of Rei's and Set's circumstances, since he basically told Well to run away 
Comment by Lady.Tistla on Legend (Takano Masaharu) - Chapter 36: v2
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I think he did seed of caution ceremony to prevent them from leaking information about him and his griffin power.