Out favorite shadow's of shadows the Drazi strike out for symbolic revenge and attempt to destroy the earth on the 5th anniversary of Liberation day over the shadows.

The shadow cloud that would wrap a planet in a net of nuclear missile firing stations was destroyed by the new mega destroyers

As a consolation prize the Drazi drop a misconfigured plague on the planet that will kill everyone in 5 years.

This is all setup, no payoff... I think its to establish the premise for the TV series Crusade.

As far as planet killers go, relativistic rocks going through hyperspace could destroy a planet without all the fuss... or jumping into the center of a planet. The Shadow Net is way overdramatic.