The Reverseworld is one of the many Evil LNH universes, based off the Classic LNH.
An evil Ultimate Ninja, along with several evil LNHers, had taken over the Legion, killing the heroic LNHers who had tried to stop them, putting their heads on spikes (ew!), and wreaking havoc and chaos as the Legion of Net.Villains (no, not that one). Decibel Dude and Vigilante Guy went there during Retcon Hour to bring back the greatest hero of that world, Captain Oblivion. A version of Deja Dude from this world also attended a meeting of duplicates in Looniverse-A.
It was eliminated by wReamicus Maximus when he purged the alternate timelines and collapsed the Omnilooniverse down to a single point, but was presumably restored when Continuity Champ Junior took the Ring of Retcon and undid wReamicus's changes. The Reverseworld Legion of Net.Villains may have been one of the 666 evil LNHes that showed up to try and destroy the Looniverse-A LNH in the great battle royale in Beige Midnight.