This module is not so great. It seems that authors have no idea about what a T4 challenge should be like. I had to rewrite all the encounters in order to provide my players experience they want.

And the unlocked reward is... horrible. Gate spell is useless in 5e and especially in one-shots.

P.S. I would higly recommend to defend the area with the fey with a permanent druid grove spell for providing additional challenge.