Tiamat's Frigid Greatsword

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails. If the weapon does not already have the reach property, it gains the reach property for this attack. On a hit, the target suffers the attack's normal effects, and a shard of ice is left within them. It explodes, forcing each creature within 5 feet, including the target, to make a Dexterity Saving Throw. On a failure, a creature takes 1d8 ice damage.

At 5th level, the melee attack deals an additional 1d8 ice damage to the target, and the explosions damage increases by 1d8. Both damage rolls increase by 1d8 at 11th Level and 17th Level.