Boon of the Slasher (5e Epic Boon)
From D&D Wiki
Boon of the Slasher
When you attack a creature with a weapon that deals slashing damage and you roll a 20 on the attack roll, you lop off one of the target's limbs, with the effect of such loss determined by the GM. A creature is immune to this effect if it is immune to slashing damage, has no discernible anatomy, has legendary actions, or the GM decides that the creature is too big for its limbs to be cut of with the weapon. Such a creature instead takes an extra 4d6 slashing damage from the hit.