Amputate

You coat your hand with powerful Osteomancy magic, make a melee spell attack roll against a creature, on a hit you deal 10D8 slashing damage. In addition, you rip off one of the creatures limbs (leg or arm).  A creature is immune to this effect if it is immune to slashing damage, has legendary actions or the DM decides the creature is too big for its limb to be cut off with this spell. Such a creature takes an additional 5D8 slashing damage instead. 

At Higher Levels. When you cast this spell using a spell slot of 8th level or higher, the primary damage increases by 2D8, and the secondary damage by 1D8 for each slot level above 7th.