Something simmilar to the roar twins elemental lasers. Firing would render the monster immobile, and the beam would stay for a certain period of time. It would have to be smart enough to not simply follow the player all the time, it would have to attack from different directions in order to restrict the players movement. It would also be smart enough to perhaps shoot where the player is going, in order to focus on sealing off escape routes and letting other mobs run into players, as opposed to just directly hitting knights. They would have to have some other attack though, since the immobility caused by the beam would make it vunerable and ineffective on its own.