SPOILER:
You need these things:
1.- A scroll, found in the Shadow Dragon (Thaaxxxi´lla or something like this). Its the Dragon you find in Umar.
2.- A thunderbolt hammer, found in the sewers under the temple district. You need first to go to Windspear hills, kill Tazok, and with the key you find in his body go to the sewers and open the secret door located in the north part of the sewers
3.- Gauntlets of ogre power. I cant remember where can you find this ones
4.- A girdle of frost giant strength, found in the passage out from the underdark.
When you have all these items, you should go to Cromwell (in the Docks district) and he´ll forge it for you.
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"Let my spells do the dirty work" Ertai, High Sorceror of the Holy Flame