Omega-Level Spells (3.5e Other)
From D&D Wiki
An Omega-level spell is one that has the capacity to destroy or negatively affect a very large area, usually the size of a country, or possibly an entire planet. Common types of Omega-level spells include bringing down fire from the sky over a sizable region, calling in an extremely large and dangerous being from another plane, invoking a flood that deluges the world, covering the globe in darkness, or spreading a plague across an entire continent.
Typically, an Omega-level spell cannot be completed by a single individual over a short period of time. Mostly, harassing the power needed for such spells requires the collaboration of many high-powered mages and/or clerics over a long period of time, often using very specialized equipment, ingredients, ceremonies, and potions. Sometimes, the knowledge of how to invoke such a spell only exists in an ancient tome, or the spell has to be invented step by step. On occasion, making a mistake in casting such a spell can lead to painful or lethal consequences for the casters, or else making a single slip-up can force the entire casting process to begin again.
 Examples of Omega-Level Spells
Apocalypse, Incinerate All (Alignment)