Sword of Sharpness

From Dungeon Master's Guide, page 206. Found on Magic Item Table H.

Description
Weapon (any sword that deals slashing damage), very rare (requires attunement).

When you attack an object with this magic sword and hit, maximize your weapon damage dice against the target.

When you attack a creature with this weapon and roll a 20 on the attack roll, that target takes an extra 14 slashing damage. Then roll another d20. If you roll a 20, you lop off one of the target's limbs, with the effect of such loss determined by the DM. If the creature has no limb to sever, you lop off a portion of its body instead.

In addition, you can speak the sword's command to cause the blade to shed bright light in a 10-foot radius and dim light for an additional 10 feet. Speaking the command word again or sheathing the sword puts out the light.