Clingy Annihilator

The Clingy Annihilator is the ultimate weapon. It is aimed at a whole enemy planet rather than at one baron, and your planet can only have one at a time.

Nobody buys the weapon alone. One baron starts the work and names the target planet. After that, any baron here can put money in, a million gold at a time. The price depends on the size of the target planet: aiming at a planet much larger than yours costs more.

When the money is all in, the weapon is ready. Only the baron who started the work can launch it. They can scrap it instead, but the gold is spent either way.

The target planet can see the weapon coming. They are told while the work is going on, and told again at launch, with the hours left before it lands. Only jets can reach it. Every wave takes a piece out of the weapon, and enough waves destroy it in the air.

Whatever survives the flight strikes every realm on that planet and takes a share of their regions. An SDI program is no help against it — jets are the only thing that can reach the weapon.