In
NetHack game terms, Stormbringer is a
runesword that deals 2d4
damage to "small" creatures and 1d6+1
damage to "large" creatures. It magically bestows an additional 1d5
damage and a 1d2 to-hit bonus.
Wielding Stormbringer confers
resistance to
level drain.
Stormbringer is an intelligent weapon and highly chaotic. This is manifested by removing the confirmation question for attacking peaceful or tame creatures -- the sword just hauls off and slashes at everything you point it at. Although the practical effect of level drain on NetHack monsters is to remove 1d8 hit points (permanently), these are not bestowed upon the wielder as of version 3.3.0.
You can receive Stormbringer by wishing for it, offering a lot of corpses to your god, finding it lying around in an Antique shop (infinitessimally small chance), or being crowned the Glory of Arioch by your god.