Celestial Fury

The Celestial Fury was a powerful katana designed to slay a dragon but ended up housing its spirit.

History
The province of a waan daimyo, Hoshi Katahiroi, was suffering under unnatural and destructive weather. A dragon of a type never encountered before was found to be the cause - it was dubbed Celestial Fury. Hoshi wished the dragon dead to spare his province more devastation. In his lands resided a reclusive weaponsmith called Chigatta who was also known to be a master of the mystic arts. Hoshi commissioned Chigatta to forge for him a weapon that could destroy Celestial Fury's spirit as well as its body.

Chigatta created a katana more fine than Hoshi could've imagined. The sword was imbued with the power of flight and so Hoshi took to the skies to slay the dragon. Above the stormclouds, roars echoed along with a man's shouts and fire lit up the night skies for several days until Hoshi returned to the ground claiming a victory of law over chaos since the dragon's spirit was now trapped inside his sword. Hoshi then asked Chigatta to craft a scabbard next and used a magical pearl that was in the dragon's possession to ornament it.

Unfortunately, while Hoshi was a great warrior, he was weak of will and the dragon's spirit possessed Hoshi's body, forcing him to kill first Chigatta, then himself. The blade disappeared shortly afterward.

Powers
Celestial Fury was a +3 sentient katana. It would only allow itself to be wielded by a chaotic-aligned individual - all others were electrocuted when they touched it.

Just having Celestial Fury in one's possession provided immunity to all natural and magical electricity damage as well as preventing ones person and possessions from becoming damp from precipitation.

Drawing the blade from the scabbard caused a flash of lightning and rumble of thunder that could temporarily cause both blindness and deafness in all but the wielder. The wielder would also generate a two mile radius of inclement weather centered on themselves, regardless of local weather conditions.

Every time the wielder landed a blow with the sword, it would emit thunder again, though not quite as loud as when it was drawn, it would still cause deleterious effects upon the victim. Three times per day, a stroke could electrocute those it hit and once per day, the sword could evoke a lightning bolt.

The sword still retained the power of flight imbued by Chigatta as well.

The dragon's sentience within the katana was a particularly chaotic one. It desired its wielder to display exhibitions of their power and enjoyed its ability to cause bad weather. Beyond this, it desired vengeance against the descendants of Daimyo Hoshi and Chigatta for taking away its freedom but could only communicate through a form of vague empathy.

The Celestial Fury was an enchanted katana found in Baldur's Gate II: Shadows of Amn. Even though it was the most powerful such weapon in that game, its history was not included until the Enhanced Edition was released. It had a +3 enhancement bonus, could cast Blindness and Lightning Strike, and had a chance to stun and do electrical damage to opponents hit.