A darkness orb is an uncommon artifact. Darkness orbs use a simple sustain rune to keep a darkness spell active within it. The orb can be activated by casting darkness into it, and deactivated by casting darkness into an active one. While active, the darkness orb will absorb light from all directions, thus dimming a 10 meter sphere.
Darkness orbs are used by criminal organizations to hide activities, especially in alleyways. If you see an alley that looks unnaturally dark, it's probably best not to investigate.
A similar object is a light orb.