The Illusionary Vulpin enemy that spawns as a part of the Mistcaller encounter does a cast to prime its ability to hit a player and turn them into an ice block.
If you grip the mob during this cast, you can prevent it from doing its only ability.
Likely not intended, as no other disruptions work like stuns or other knockbacks.
Video showing it in action:
https://www.youtube.com/watch?v=Y1sG4pbmFlc