A shard made of black crystal that the group has at the beginning of the campaign.
EDIT – Where did they get it from?
The shard gives it’s wielder the Swirl of Darkness power, which works as follows:
Encounter, minor action Makes a wall up to five squares long, starting from your square. The wall blocks line of sight and the squares within the wall are completely obscured. Creatures within that try to go through the wall or are within the wall suffer from blindness (save ends). None of these things affect you. The swirl is also difficult terrain for all creatures except you. Sustain Minor