The Swarm Invasion

The Swarm attacks both the Pact Worlds and the Veskarium, prompting an alliance.

After more than 200 years of conflict, the Silent War between the Pact Worlds and the Veskarium is interrupted when the Swarm invades both systems. In order to survive, the two governments agree to end their hostilities, working together to successfully rebuff the Swarm’s mindless destruction. Source: Starfinder Core Rulebook pg. 427.