Quake
The story of the game involves Lovecraftian creatures, led by Shub-Niggurath, coming through "slipgates" (teleporters or portals) to invade this dimension. Evidently, the whole experience was so shocking for the Strogg, they decided to research bionics in order to turn themselves into superior beings and repel eventual future invasions more easily. In time, this got out of hand, and bionic enhancements became an end onto itself, instead of a means to reach an end. Alternatively, Sub-Niggurath's influence may have subtley corrupted the Strogg, leading them to take their research too far.
Ever notice the military crates in Quake 1? They have the Strogg symbol on them. This means that in Quake 1, the "good guys" are Strogg! And, more specifically, Quake 1 is set centuries before 2 and 3, when the Strogg were simply a human-like race, without any experience in bionic implants yet.
- To some extent, it can be argued that every game is connected in some way or another. The Quake timeline may as well could be Quake I-[ET Quake Wars-Quake II-Quake IV]-Quake III Arena.
Building on this, in Quake, the humans are researching Slipgate technology. What did the Strogg use to attack Earth in Quake Wars? Slipgates. Ho hum.
2) There is a faction called the Crusaders in both The Elder Scrolls II: Daggerfall and Quake III: Team Arena. The Crusaders in Daggerfall are the militant wing of the Temple of Stendarr, while the Crusaders in Team Arena are a group of religious types in medieval armor. And if that isn't enough, the Crusader named James makes references to The Elder Scrolls V: Skyrim in his taunts in Quake Live. So it's likely that the Vadrigar might have abducted members of the Crusaders from Tamriel at some point, and the ones we see in Team Arena are a distant off-shoot of their Tamriel counterparts that has fought in the Arena Eternal for so long that their religious beliefs have been muddled and confused to the point where they no longer practice what their temple preaches; How ironic, that worshipers of the God of Mercy would end up fighting in a tournament where battles to the death are mandatory.
3) The Elder Gods and the Old Ones, two groups of deities originating from the Cthulhu Mythos, seem to exist in both Quake and The Elder Scrolls. The Elder Gods are prominent in the lore of Quake Champions, and are mentioned in a rumor in The Elder Scrolls: Arena. Shub-Niggurath from the original Quake is classified as one of the Old Ones, which are also briefly mentioned in a Daggerfall book titled "The War of Betony", by Fav'te.
- Some Reality Subtext for the whole entry, as ever since 2009 id Software became a part of ZeniMax Media, which also owns Bethesda Softworks. Not to mention, before leaving Id, Tim Willits (the last of the Id originals) wanted the Dragonborn as a playable character in Quake Champions.