A Rebel Rush event has just begun for the Half-Life 2 mod Synergy. These events do not come often so use this opportunity to grab a very limited achievement.

IMPORTANT: you'll want to opt into the beta branch for Synergy.

You should be able to join, grab the achievement (it should pop a few minutes after joining the lobby) and then leave to let more people in to do the same.

You can connect using this link: https://hgrunt.xyz/connect_syn
Or by using the console:
The event is hosted by the Half-Life discord, and you can find more details there https://discord.gg/ySBPspFM?event=1175995041609699388