http://www.moongates.com/images/Ultima7fvbox.jpg http://vnmedia.ign.com/nwvault.ign.com/fms/images/nwn2models/109/1192335930_fullres.jpg http://images4.wikia.nocookie.net/__cb20070108132243/u5lazarus/images/4/44/Bswrd.gif
Spoiler
"The Black Sword is an enchanted two-handed sword. Its blade is metal, but the hilt is made of blackrock, a physically unmalleable material. Blackrock is not very easy to shape, therefore powerful magical enhancements are required to make the weapon properly cut and balanced. The most important of these is a magical soul gem embedded in the hilt.
In Ultima VII, the Black Sword is magically bound to the Avatar, and cannot be unequipped. When the sword is activated in the game by double-clicking it, the Avatar is able to converse with the daemon Arcadion, who is able to either instantly and noiselessly kill a creature, fill up the Avatar's mana, create a fire field or teleport the party to the Isle of Fire. Arcadion can kill creatures that are very powerful, but refuses to kill creatures that are far too easy to kill by conventional means. Some creatures, like liches and stone harpies, he can't kill. Using the Black Sword is one of the few non-easter egg ways to kill Lord British. Killing Batlin with it is not possible, however."
In Ultima VII, the Black Sword is magically bound to the Avatar, and cannot be unequipped. When the sword is activated in the game by double-clicking it, the Avatar is able to converse with the daemon Arcadion, who is able to either instantly and noiselessly kill a creature, fill up the Avatar's mana, create a fire field or teleport the party to the Isle of Fire. Arcadion can kill creatures that are very powerful, but refuses to kill creatures that are far too easy to kill by conventional means. Some creatures, like liches and stone harpies, he can't kill. Using the Black Sword is one of the few non-easter egg ways to kill Lord British. Killing Batlin with it is not possible, however."
So it should be powerful enchanted two-handed sword which has ability to kill creatures in one hit. Maybe add an enchantment which drains loads of the targets health when used. The enchantment should have only one use when it's fully charged.
I noticed that http://images.uesp.net//9/95/TB-Weapons-Trueflame.jpg handle looks very similar to the black swords handle, so maybe it could be used when making the sword?