Caustic Strike

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 the 5 foot square beneath the creature becomes a puddle of acid. A creature who enters the area of the puddle or ends its turn there must succeed on a Constitution saving throw or take 1d8 acid damage.

At Higher Levels. At 5th level, the initial melee attack deals an extra 1d8 acid damage to the target on a hit, and the puddle's damage increases to 2d8 acid damage. These increase again at level 11 (2d8 and 3d8) and at level 17 (3d8 and 4d8).