Orion's Threat, included in Alien Swarm: Reactive Drop, was formerly the only campaign for which it was impossible to acquire all campaign-specific achievements. However, the first post-release update fixed the impossible achievements, and they can now be earned.
Stop an Alien Swarm!
- Destroy Swarm Eggs 0/23
- Redirect power
- Recieve [sic] Code
- Proceed deeper inside the facility
- Destroy Swarm Eggs 0/18
- Access the industrial complex