After shooing the Dark Space Bear away from the Mystic Forest's edge area, Alister walked to the Sea Of Chaos.
As much as he wanted to spend several decades with Zanna, Alister knew that his priority was to become as strong as possible.
He wanted to use the Trinity (Red) talent to create an avatar so that he could manually control it to protect Zanna while he was away. However, that wasn't possible since the avatar was a fragment of his soul.
All of his soul must be tempered by the Chaos Body and if one soul fragment wasn't tempered along with his soul, then the tempering would be incomplete. Not only would it hinder his progress in increasing the attainment percentage of the Chaos Body, but there would also likely be some side effects that he was unaware of.
Alister didn't want to take that risk.