Superweapon

A superweapon is a UFO item which can cause massive destruction on a planet. Only three superweapons are known as of now.

Planet Buster
In the 2005 GDC Presentation, Will demonstrated a weapon on his UFO capable of destroying an entire planet, turning it into an asteroid belt. Also known as the Planetbuster (a reference to Sid Meier's Alpha Centauri) or Death Ray. There will probably be a way to destroy a star, and then the entire solar-system.

Genesis Device
The Genesis Device works by turning a planet's atmosphere (or lack thereof) into one that is perfect for your species. However, if an enemy species could not breathe your atmosphere, the Genesis Device would essentially choke the entire planet, with the possible exception of Domed Cities.

Atomic Bomb
It is possible to build a a-bomb what destroy an entire city, and many can destroy an planet's population. it is unknown if it will be possible to build in Civilization Phase, and if it is so, it maybe is weaker than the UFO's version.