Black Ice Blade (Bladespell)

You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within the spell's range. On a hit, the target suffers the weapon attack’s normal effects and must make a dexterity saving throw or fall prone as a thin layer of ice forms beneath them. If the attack misses then the target makes the saving throw with advantage. 

A creature more than one size larger than you cannot be knocked prone by this effect.