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, takes 1d6 cold damage and then a single limb becomes encased in thick frost until the start of your next turn. If an arm is encased then the target cannot make opportunity melee attack actions. If a leg is encased then the target's walk/run/swim speed is reduced by 5 ft. If the target creature has other appendage options (tentacle, tail, wing, eye stalk) the DM can make discretionary changes based on what it is (treat a tentacle like an arm, a wing would reduce fly speed, or similar).
This spell’s damage and effects increase when you reach certain levels. At 5th level, the damage increases to 2d6 cold damage to the target on a hit. At 11th level the damage increases to 3d6 cold, and two separate limbs can be encased. Options are two arms (cannot take any reactions and disadvantage to any grapple actions), 1 arm/1 leg mixed (both of the beginner options) or two legs (walk/run/swim speed is reduced by 10 ft). At 17th level the damage increases to 4d6 cold.
* - (a melee weapon worth at least 1 sp)