Introduction This is a guide to the Omniverse, the Captain Britain Corps, Opal Luna Saturnyne and the related and sometimes confusing characters and storylines that started appearing in early Marvel UK titles featuring Captain Britain and later the prominent Excalibur series.
The Omniverse In the Marvel Comics there exists more than one reality, more than one universe, and more than one Earth. In fact, there is a seemingly unlimited number of parallel dimensions in which some events played out different than in the mainstream universe, creating an alternate reality. Some of these other realities may seem very similar to the known universe, others differ substantially. All of these universes together make up the Omniverse. The multitude of alternate Earths / realities are traditionally watched over by Merlyn and his daughter Roma from Otherworld. They named Opal Luna Saturnyne as Omniversal Majestrix to govern all these universes and oversee their progress. For a short time, Saturnyne was replaced by her second-in-command Mandragon and, more recently, Saturnyne herself has replaced Merlyn and Roma.
Chosen champion and the number "616" Each of the multitude of Earths has its own number; the mainstream Marvel Universe is called Earth-616, as opposed to the Ultimate universe (Earth-1610), the Days of Future Past-timeline (Earth-811), or the Age of Apocalypse (Earth-295). In each reality, Merlyn and Roma chose a champion to safeguard the area of Great Britain. The Omniversal Energy Matrix, a vast power source Merlyn helped create and relied upon, was situated in that area regardless of the Earth. On Earth-616 the chosen champion was Brian Braddock, aka Captain Britain. In fact, on a statisitically improbable number of alternate Earths, the chosen champion was that Earth's version of Brian Braddock. On rarer occasions, it might be a counterpart of another known Marvel character (Psylocke, Meggan, Alysande Stuart,...) or someone completely new. In most incarnations the champion's title is somehow connected to Britain, England or any other words to refer to the area.
Captain Britain Corps All of these chosen champions make up the Captain Britain Corps. They have to obey a certain moral code (each's defined by the laws of their native world) and those who violate these rules are put on trial. Technically, "Captain Britain Corps" is the local designation for Merlyn's champions, named after Earth-616's champions. Other Earths might refer to the group locally as the Captain UK Corps, the Kaptain Briton Corps, or simply "the Corps" for general use.
Only those champions that were chosen by Merlin or Roma belong to the Corps, but not people who replaced a Captain for some time using his/her amplifying costume. For example Psylocke (616) took her brother's place for a short while and was known to the British public as Captain Britain II, yet she did not belong to the Captain Britain Corps at that time. Somewhat confusingly, versions of Captain Britain can emerge in the multiverse that have nothing to do with Corps. At the same time, Merlyn and Roma occasionally use disguises or work indirectly to empower their chosen champions before eventually inducting them officially into the Corps (as happened with Brian Braddock). Therefore, any Captain Britain seen throughout the multiverse may not be part of the Captain Britain Corps, but might also BE a novitiate of the Captain Britain Corps and not yet know it.
Traditionally (but not always), Merlyn and/or Roma anoint their champion by giving them the choice of talisman between the Sword of Might and the Amulet of Right. The "correct" choice is the amulet, justice over violence, and all members of the Captain Britain Corps supposedly made that choice. Some do choose the Sword, however, and the full ramifications of that choice are unclear. Albion was allowed to retain the powers of a champion, yet was not inducted into the Corps. Kelsey Leigh received the powers of Captain Britain, yet suffered a curse that meant her children would continue to believe her dead and she was helpless to correct them.
Saturnyne and her counterparts On Otherworld, Opal Luna Saturnyne functions as the Omniversal Majestrix and guards all the alternate Earths, yet she was born on Earth-9, which means she somehow must have acquired that position at one point in her life. For a short while she was replaced by Mandragon, her second-in-command, but later she blackmailed her way back into her former job.
Like Captain Britain or any other Marvel character, Opal too has counterparts on other Earths. On the mainstream Earth (616) this is Brian's former girlfriend Courtney Ross, who became a prominent bank manager. The other well-known counterpart of hers is Opal Lun Sat-yr-9 (from Earth-794); she was the dictator of Britain under a tyrant regime. Later Sat-yr-9 escaped to Earth-616, where she killed Courtney Ross and took her place for quite some time before she was exposed and fought by Excalibur.
Besides these two there some other counterparts seen, mostly during the "Cross-Time-Caper". Possibly Sat'neen of Earth-148 (the true love of later Excalibur member Kylun) is a counterpart too as the name suggests.
Excalibur's Lighthouse - a nexus place Excalibur's lighthouse (or actually its outer walls) is a building that simultaneously exists on every Earth throughout the multiverse, representing the Omniversal Energy Matrix. In some realities the lighthouse is indeed a lighthouse, in others it functions as a tower or even as a pillar of a giant hall. Because of this nexus' nature the walls between the dimensions are very weak in and around the lighthouse and sometimes Excalibur's members suddenly saw themselves face to face with beings from alternate realities. Yet mostly the effect vanished again. Only once or twice beings actually crossed over.
Final Note The "616" label for the Marvel Universe has proven highly resilient. In X-Man #71-74 a villain named Qabiri was running around destroying Earths. The new terms "612" or now "611" related to the "former 616" reality and meant to imply that Qabiri already destroyed five Earths that had lower numbers than 616 and all the numbers moved up. Qabiri's hubris was never supported, and the Earth-616 label remained. Despite the conventional and geocentric label "Earth-###", the term actually referred to entire dimensions. Qabiri's efforts merely meant that several universes now exist with no Earth in them.
The destruction of the Seventh Cosmos depicted at the end of the Incursion crisis in Secret Wars wiped out the entire Omniverse. Earth-616 was reborn as "Earth-Prime" at the onset of the Eighth Cosmos, and other pre-existent realities were slowly recreated as well by Franklin Richards and Molecule Man reseeding the multiverse. Despite this, the Earth-616 label has stuck, and Otherworld stories continue to refer to it by the numbering invented by the previous universe.