...One of you Winx...
The Detention Dimension is a place where students from Cloud Tower are sent for detention. It is inhabited by Blackboard who oversees the detention assignments. The way to be brought to the detention dimension is if Griffin uses the spell "Demeritus", opening a portal under a students feet sending them to the dimension.
In “Fury!,” Griffin sends the Trix here, where they are instructed to write on the blackboard six hundred ingredients that can be used in mayhem potions. Stormy and Darcy eventually succumb to the assignment but Icy still refuses. The blackboard threatens to report her behavior to Griffin. Then, Darcy uses a spell to complete their assignment and set them free.