On the Altar in the Temple of Mara you'll find the ring of protection.  Steal it but don't get caught or there will be a lot of dead citizens as a consequence. 

Version 6:  The formerly invisible horror is now visible as requested, with a couple of minor additional tweaks to keep it interesting.   Bloodshot eyes, eyes floating in space!  And a dark, sinister, cloud-like body will appear. 

Version 5:  A foul, murderous, invisible, creature of unspeakable horror will follow you whenever you wear the ring.

The invulnerable and impassive unspeakable horror will follow you whenever you wear the ring.   Once summoned be careful  where you remove the ring. Free from your control it may go on a bloodthirsty rampage.   It exists only to kill and kill again!  Is the  ring cursed, or a blessed, you decide.

Included are modified sound files to add a little gristle to the kill move.  I don't think this mod breaks anything.  At least I haven't found anything broken by it but who knows.  Remove the ring and wait for it to leave before any brawls.
 
Known bugs, or are they features...
If you ask a bard to sing it will probably kill the bard or whoever is closest to it at the time.

Requires only Skyrim and the official update.