Orcadus, known on Earth as Hell, is a dimension of dark power that supposedly houses the armies of the Demonic Emperor Dhrell. While Dhrell's kingdom has existed for tens of thousands of years, the dimension was undiscovered by humanity until the Middle Ages, when Dhrell attacked Earth but was forced back to his home by Dr. Darke.
The portals between dimensions are dangerous and incredibly unstable, so Dhrell's attempts to conquer Earth proved fruitless for hundreds of years thanks to the heroes of Earth, including The Knights. It wasn't until Dr. Darke forced open the portal and invaded Orcadus that Dhrell stood a real chance of invading and conquering Earth. After the events of Darke Day, Dhrell and his armies were locked away from Earth and have remained dormant since.
Other demons have occasionally made their way to Earth, either accidentally or by way of magic.