Nethack artifact.
It's a long sword, with the special ability to freeze the opponent. It also (logically) provides cold resistance (fire resistance in Slash'EM). It has +5 to hit, x2 damage.
This artifact is not too mighty... get a better one if you can.
The artifact has no alignment and will be in bones files, you may get it by offering, may be randomly generated and can be wished for.
See also Fire Brand.
(Source: Kevin Hugo's spoiler files, updated by Dylan O'Donnell. Current as of Nethack 3.3.0.)