Verglas Blade

You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects and then becomes wreathed in a thin layer of frost, the target's movement speed is reduced by 10 feet. 

At 5th level, the melee attack deals an extra 1d8 cold damage to the target on a hit. The damage roll increases by 1d8 at 11th level (2d8) and again at 17th level (3d8).