The Order of the Midnight Sons was a "band of brothers" based on an ancient order of the same name. The modern incarnation arose from the Mystical Nine who were destined to defend humanity against the rise of Lilith, Queen of Monsters, and her demonic children. After the defeat of Lilith, the remaining members of the Nine and their allies remained loosely associated with one another as the Midnight Sons.
Years later, Morbius of the Midnight Sons rebuilt the team with new members to oppose the zombie plague that crossed over to our reality. These Midnight Sons were connected with ARMOR and the Florida Command. Since then, the Midnight Sons have generally included groups of mystically-inclined heroes, often those whose souls are already tainted or damned for some cause.
Other members unrelated to the X-Men include Ghost Rider, John Blaze, Vengeance, Morbius, Hannibal King, Blade, Strange, Doctor Strange, Werewolf by Night, Jennifer Kale, Modred the Mystic, Doctor Voodoo, Moon Knight, Iron Fist, Man-Thing and Wong.