Continual Darkness

The caster can imbue an item with a sliver of the Shadowfell. This causes a 15-foot-radius of darkness centered on the item. If cast multiple times only the most recent last until dispelled. Earlier casting shrink as times goes on at the rate of 5' per day. For example, if this spell is cast on two items, on day two one is a 10' sphere of darkness the other is still 15'. 

Using a higher level spell slot increase the radius by 5'. So using a 4 level spell slots creates a 20 foot radius sphere.