Eryndor waited for the disciple to unleash the killing strike upon the beast.
People were alert during the fight, but not so much when they think they've defeated their enemy.
Eryndor had already deployed his concealment cultivation art.
It could mask his sound, breath, body heat, and also make him invisible.
It was only now when the situation was appropriate for him to ambush that he realised he had bought a concealment cultivation art not too long ago.
Jeez, how did I forget about it?
He could have used it to ambush Jaxen if he remembered it back then.
There was also the fact that Jaxen's situation was not like now, where the disciple's full concentration was on the beast, and the fight was intense despite the disciple being on the winning side.
Jaxen was completely relaxed, and was simply torturing the flying beast. No matter what, it couldn't have evoked the thought of ambush into Eryndor.
