The jar of darkness is a rare drop from Skotizo. Players can right click the View option on the jar to have the game screen go black for a brief moment, with the chatbox stating You view the darkness.
The jar can be used on a boss lair display in a player's Achievement Gallery to have a display of Skotizo. Doing so requires killing it at least once.
The drop rate of the jar of darkness is exceptionally low relative to the time required to obtain dark totems, taking approximately 4,000 hours of Slayer training within the Catacombs of Kourend (hypothetically gaining 400 million Slayer experience) to reach the average rate of obtaining one. This was considered an oversight by Mod Arcane during a weekly Q&A session, though Mod Roq, who developed the content, stated that he "liked [the rate] where it is". Ultimately, the drop rate was polled to be increased from 1/2,500 to 1/500, but failed by 3% of the required votes.
For an exhaustive list of all known sources for this item, see here