The Shadow Shard Universe (or simply the "Shadow Shard") is an alternate universe in the City of Heroes MMO.
It is an area where the rules of physics are twisted and warped. Where water runs like blood and gravity can be manipulated. Strange beings exist here that defy our normal understanding of reality and sanity. Some portions of the Shadow Shard are habitable. Others are not.
This is the realm of Rularuu the Ravager, a deity that has absorbed thousands of multiverse realities and seeks to absorb the rest so it can become all-powerful and all-knowing.
Once upon a time, this was the prison for Rularuu. His quest to absorb the Primal Universe in the 1960's was stopped by a secret team of heroes called the Midnight Squad, and they sent him here, where the rules of this realm would work against him. And for a time, it did. But eventually he prevailed again and he learned how to gain control over even this realm.
This is also the home of the interdimensional nightclub known as Pocket D, along with the nearby Ski Chalet, which has provided a neutral area for heroes and villains, and, thanks to a secret door to Studio 55, even people from the Praetorian Universe can meet, drink, and have fun. The ravages of this part of the Shadow Shard are too extreme for even some superhuman beings, which is why these places are protected by thick walls and protection barriers.
(See its Paragon Wiki entry for more information.)