The Dark Ones guard the passage up Mount Draak in the center of Portabello. They guard the Starfly Key, which Zaggle sends players to recover in order to free the Starflies at the mountain's summit. Dark Ones only have 2 Life and their melee spin attacks inflict only 1 damage to players, making Dark Ones easy to defeat.
Dark Ones originated as Hyper Darklings, enemies found in the pre-alpha versions of Gnarled Forest and Forbidden Valley. Like other Darklings, Hyper Darklings would have been spawned by infected shards flung by the Maelstrom and crashed into the Worlds. Hyper Darklings would have continuously spawned from these Maelstrom shards until the shards were smashed by players. If a Hyper Darkling was smashed, it would have reassembled itself after a few seconds.
When the worlds were redesigned, Hyper Darklings were removed from Gnarled Forest and Forbidden Valley. Like many other pre-alpha elements of LEGO Universe, Hyper Darklings were reused in Portabello as Dark Ones. However, while they share the same model, Dark Ones have different programming from the original Hyper Darkling; for example, Dark Ones do not reassemble themselves once smashed.