This appears as a well-crafted metal torch with magical runes on it. If deciphered, the runes reveal a keyword. Upon testing, it will be learned that if the torch is held by someone, and that person speaks the keyword, it will ignite and burn like a normal one, but never burn out. Only if the holder of the torch speaks the phrase again, will it go out. The torch cannot be snuffed out, and will continue to burn when wet, or even under water. It will even burn and throw light in the midst of magical Darkness.
The curse reveals itself the first time the torch is in possession of someone in the midst of combat, whereupon it will sputter, flare up, and finally explode as a 6d6 Fireball, centered on the holder. After the explosion, it will be found sitting on the floor, glowing red hot, but otherwise unharmed. When it cools down, it will be ready to use again.
Optional: There is no Saving Throw allowed for the torch holder, though they'll have one chance to toss the torch away. If they do, the flare-up will cease, and the torch will appear to return to normal. It will try to explode again, if held during another fight, whether it's ignited or not.