Stormbringer

From Wikihack

Stormbringer is a chaotic artifact, as well as the only runesword normally generated in the game. It is considered a broadsword for skill purposes. It drains life from monsters it attacks ("The black blade draws the life from <monster>!"), healing you for a few hit points per attack. It is bloodthirsty, attacking peaceful monsters and pets without confirmation if you attempt to move onto their square. ("Your bloodthirsty blade attacks!")

The damage and to-hit bonuses are applied only against creatures not resistant to level drain.

Stormbringer is a great melee weapon for Chaotic Rangers. As soon as you get it, you can dump your Sting. A +7 Stormbringer will easily clear out hordes of monsters in places like Fort Ludios or the Astral Plane. It will also be an important weapon in Gehennom where projectiles can't be reliably used if the area isn't mapped out.

Once you get this sword, don't even think about owning pets. You will kill them before you know it and you will suffer the consequences. The rule of the thumb is if you have Stormy, don't tame anything.

[edit] Encyclopedia description

       There were sounds in the distance, incongruent with the
       sounds of even this nameless, timeless sea: thin sounds,
       agonized and terrible, for all that they remained remote -
       yet the ship followed them, as if drawn by them; they grew
       louder - pain and despair were there, but terror was
       predominant.
       Elric had heard such sounds echoing from his cousin Yyrkoon's
       sardonically named 'Pleasure Chambers' in the days before he
       had fled the responsibilities of ruling all that remained of
       the old Melnibonean Empire.  These were the voices of men
       whose very souls were under siege; men to whom death meant
       not mere extinction, but a continuation of existence, forever
       in thrall to some cruel and supernatural master.  He had
       heard men cry so when his salvation and his nemesis, his
       great black battle-blade Stormbringer, drank their souls.
               [ The Lands Beyond the World, by Michael Moorcock ]