Crom Faeyr +5, which is also the most powerful weapon in the game. You'll need to put it together using the following:
*minor spoiler*
Hammer of Thunderbolts +3
Girdle of Frost Giant Strength
Crom Faeyr scroll
Gauntlets of Ogre Power
Once you have all these you'll need to take it to a smith named Cromwell, in the Docks district.
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