Amberser did not take Bartle and the other Mercury Avatars with him, he merely healed their injuries and then allowed them to hide and wait for his command.
This natural Mercury River was the most suited environment for their growth; as long as they sank to the bottom of the river, ordinary demons would not be able to do anything to them.
Amberser's initial plan was for Bartle to stay here to observe the movements of the demons, and when necessary, he could always use the Magic Mirror to tinker with this spatial passageway.
But Bartle had a different idea.
As a loyal follower of Amberser, he inherently regarded Amberser's wishes as the goals for which he strove.
Although Bartle did not know what Amberser's real plan was, he knew that Amberser aimed to confront the demons, those endlessly sprawling hordes of demons that extended as far as the eye could see.
