The Mind Flayer was introduced in season two, episode eight.
A monster from an unknown dimension. It enslaves races of other
dimensions by taking over their brains using its highly-developed
psionic powers. It wants to conquer the group and spread and take over other
dimensions.
Strengths and Weaknesses
Strengths:cold, and spying
Weaknesses: heat, and unknown places
The Mind Flayer (shadow monster) is controlled by Vecna
and wants to make Will a host so he can spy
on the group.