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. The author of the campaign asked for removal of his content from the main game after having differences with the main developer. The author continues to work on the campaign now named Adanaxis Campaign. It features 7 maps and is rated as one of the best campaigns available.
You will need a Tech in every mission.
Stop an Alien Swarm!
- Destroy Swarm Eggs 0/23
- Redirect power
- Recieve code
- Proceed deeper inside the facility
- Destroy Swarm Eggs 0/18
- Access the industrial complex
- More Eggs to Destroy
- Download data and pass Airlocks
- Immediately evacuation