The Myriad Shadowrealms is a collective term which refers to the collection of all existing Shadowrealms, including the Universe in which Earth exists.
Three ways exist to navigate the Myriad of Shadowrealms. The first is by way of leylines and leygates, in which one can travel the same Shadowrealm or multiple Shadowrealms. The second is by way of the Swords of Power, which have the ability to create leylines to any place including Shadowrealms in the past, present, or future. The third way is through Shadowrealms' individual physical gates that are all connected to one another apart from the few that were sealed off, whose inhabitants can never return to any other Shadowrealms. Dr. John Dee made use of the swords' ability to escape from his masters' minions during his time as an utlaga, or an outcast.
"I will make you the new Isis. I will make you empress of the Shadowrealms."
-Dr. John Dee to Virginia Dare
Although it is nowhere stated that Isis was actually the anointed empress of the Shadowrealms, it is obvious that she maintains a great deal of power and influence within the Myriad. Later, Dr. John Dee convinced Virginia Dare to aid him in his war against the Elders by promising her the Myriad as her dominion.
As of the end of Warlock, 9 different races are known to inhabit or at one time have inhabited the Myriad of Shadowrealms. These races are as follows:
5. Great Elders
7. Next Generation (including the monsters and creatures of magic)
9. Common animals and plants