Pathways into Darkness (1993), Marathon (1994), Marathon 2: Durandal (1995), and Marathon Infinity (1996) all precede Half-Life (1998).
https://en.wikipedia.org/wiki/Pathways_into_Darkness
https://en.wikipedia.org/wiki/Marathon_Trilogy
The stories of these games have been the subject of discussions for many years.
http://marathon.bungie.org/story/
You may know Bungie as the studio behind Halo: Combat Evolved (2001) and subsequent games.