The Ogre Magnus, is not a singular species, but a category of similar creatures that share certain characteristics. Found all over the known world, it is speculated that they share a common ancestry with one another. Despite their commonalities, they appear in almost endless varieties.
While Ogre Magnus are classified as a type of giant, they are not actually giants. Giants proper, are descended from ancient lineages with strong ties to the Elemental Planes of Power. Ogre Magnus are aberrations, infused with frightening and chaotic arcane powers. Their very existence would not be possible if not for the powerful mystical forces that permeate their very being.
All Ogre Magnus possess the ability to change into human form, even if only for a limited duration. When appearing human, they are nearly impossible to reveal without magical aid. Some individuals have lived for many years, disguised as humans, and even taking unsuspecting human wives. Eventually these long-held deceptions are discovered, either by accident or through carelessness.. at least, according to popular stories.
Legend has it that Ogre Magnus can accidentally be reverted back into their natural form, by exposure to simple circumstances particularly weakening to that specific creature. Some have been revealed after having consuming too much wine, or seeing their reflection in a silvered mirror, or upon hearing the wail of a crying infant, or tolling of church bells. The Referee is encouraged to be creative with these weaknesses, keeping in mind that each specific Ogre has only one such weakness apiece. An Ogre will attempt to flee if they feel their disguise slipping.
Curiously, there are no known instances where an Ogre Magni has ever taken the form of any other humanoid, or near-human species, other than human. Some can, and do, take the forms of common animals to move among people. These alternate forms are also subject to the Ogre's particular weakness that can force them back into their natural form.
All Ogre Magnus share certain common powers, as outlined in the Monster Section of whatever game system the Referee is using, but this does not mean that they can not possess additional powers. It is not unusual for these creatures to possess natural protections, or adaptations, that will allow them to live in dangerous and harmful environments, like the rim of a volcano, deep under the sea, in pitch black caverns, or even in castles where gravity is completely reversed and everything is upside down.
Particularly powerful, Ogre Magnus, are known as, Ogre Primus. These powerful beings will possess levels in magic-user, or cleric, with corresponding additional hit dice as well. It is not unknown for such Primes to rival Storm Giants in prowess and abilities.
Powerful Ogre Magnus, that have established strongholds, will often be served by a Djinn, or Efreet (never both). They do not accept lesser Ogres as servants, though they may allow such to serve in their armies.