Ontological Inertia is tendency of that which exists to continue existing.
In stories involving magic and the supernatural, it refers to whether or not a spell or condition continues to exist after the passing of it's creator. If killing a vampire returns the humanity to all the lesser vampires he's sired, then Vampirism can be said to lack Ontological Inertia. If a spell or curse survives it's caster's death, it can be said to have Ontological Inertia. In sci-fi and fantasy, this principle is sometimes applied to entire Empires: the death of Sauron or Palpatine results in a landslide victory by Minas Tirith or the Rebel Alliance. A lot of this will depend on how a given setting interprets the laws of magic.
In time travel stories, Ontological Inertia refers to the "force" or situation that makes it difficult or impossible for a time traveler to change history. It may occur as just a tendency for major events in history to continue on roughly the same path, or as a very potent and exact force that makes any alteration of the past impossible. It can manifest as bizarrely implausible coincidence, a more subtle thematic resonance, or even a disastrous energy release such as a Time Ripple (see also: Delayed Ripple Effect) or Marty McFly's hand dissolving while he's trying to play guitar. It may be that changing the past is a little challenging, exceedingly difficult, downright impossible, and/or highly dangerous.
Game and Story Use
- Very powerful spells could be balanced by a lack of Ontological Inertia. Kill the caster, and the spell breaks.
- When an ancient Wizard dies of old age, his greatest magics unravel, and the face of the world transforms.
- Perhaps bring the final death to all those ancient Vampires and Liches isn't such a hot idea after all.
- If the Gods created the world, will the world survive the death of a God?
- A villain could seek to collapse an Empire by desecrating the grave or relics of the first Emperor. This might or might not be a form of Sympathetic Magic.
- In a time travel campaign, the players might not start out knowing what degree of Ontological Inertia exists in the setting. They'd have to figure it out via trial and error, risking Temporal Paradox in the pursuit of knowledge.
- The existence of OI provides the cunning GM with lots of useful plot hooks - even once the villain is defeated, there are plenty of adventurs to be had cleaning up the mess he left. Without it, you need a whole new campaign.
- Lack of OI can also reduce the level of grimdark in a setting - if all (or most) of the harm done by a given villain goes away with his death, the setting is far less grim … plus in puts a very direct kybosh on the old weasly line of "killing me won't bring her back" (or whatever). Realism, of course, will suffer proportionally.