> **The Generals of Hell Have Taken Notice of You**
> 
> From:  High Command
> 
> To:  All Military Forces
> 
> Subject:  Spellcasters Interfering with Plans
> 
> Priority:  Urgent!
> 
> In response to reports from underlings about adventurer's with, dare I
> say, fiendish effectiveness against our scouts and sorties, we are
> forced to take extreme measures.  Lesser demons in need of
> disciplinary action and captured mages will now be forced to undergo a
> procedure that turns them into living Antimagic totems.  Supplies of
> these totems will be limited by our supplies and ability to produce
> them, but creative placement should make them somewhat effective.

See the **Antimagic Field** spell:
https://roll20.net/compendium/dnd5e/Antimagic%20Field#content

There are also other spells that fiends and spellcasting undead might use to counter spellcasters, such as Silence and Counterspell.  **However, be careful not to overdo it.** Part of the fun of D&D is to stomp through some encounters because of build choices you made, and you definitely don't want to completely shut down a PC's ability to contribute to a fight.