Emperor’s Blade

Spiked metal barbs line this +1 longsword, resembling the many rows of a shark's teeth. You can use a bonus action to speak a command word that activates or deactivates the barbs. While activated, the barbs rapidly saw back and forth to tear into your foes, making your weapon attacks with the sword deal an extra 1d8 slashing damage.

When you score a critical hit while the barbs are active, the sawing barbs leave a terrible wound that deals 1d8 ongoing damage. At the start of each of its turns, a wounded creature makes a Constitution saving throw (DC 8 + your proficiency bonus + your Strength modifier) to end the ongoing damage.