choose a creature within range and make a spell attack against them. on a hit they take 2d8 damage and you can expend any number of hit die and roll them adding the total to the damage dealt.

at higher levels 

if cast with a 2nd level or higher spell slot the attack deals an extra 1d8 acid damage per level above 1st in addition it deals additional acid damage equal the number of hit die expend times your spellcasting mod.