The Black Star

Wondrous item, legendary

Using this star-shaped crystal, you may imbue a nonmagical weapon with magical power.

Choose one weapon while in possession of this artifact. You may spend 10 minutes imbuing that weapon with magic, making it a magical weapon with a +1 bonus to attack and damage rolls. You may have three of these weapons imbued with magic at a time. If you imbue another weapon after the third, the first loses its magic and becomes mundane. You must wait until the next dawn to imbue another weapon with magic.

Additionally, The Black Star functions as a grand soul gem (300 gp, CR10) that does not get destroyed when used. This can hold human souls.