Quake: Mission Pack 1 - Scourge of Armagon1997

After defeating Shub-Niggurath, you arrive back at your home base on Earth, but it's not all blue skies and butterflies. Apparently, not all QUAKE forces have been subdued. Chaos ensues as you blast your way through the QUAKE minions and toward the infested gateway. Your only choice is to find the source of evil and shut it down. Without any hesitation, and with more guts than common sense, you leap into a portal of unknown destination.