I was thinking of using a trap against magic users. The idea that I came up with was basically as follows:
The victim walks into the trap area (a hallway with a width of Silence's area of effect (about 28 feet across max))
Two quite sturdy walls fall from the ceiling (~28 feet apart max)
When the victim goes to cast a spell, they discover that they can not speak and thus cannot fulfill the verbal requirements that almost every spell has due to the fact that a rune/glyph (not sure which is the correct one) casts Silence and continues to maintain the Silence spell.
My main question is: How overpowered would this trap be against magic users?
The subordinate question:
Would it be fair to pit players against this (the setting would be in BBEG's lair) trap?
How would I implement a trap like this?