Balagarn's Iron Horn

You project a 30-foot cone of vibrational energy that makes creatures weak-kneed and unable to stand. Until the start of your next turn, creatures treat the area as difficult terrain. Creatures in the area at the time of casting or that enter the area before the spell ends fall prone unless they make a successful Dexterity saving throw.

Adapted from AD&D 2e's Magic of Faerûn.