"One 4 Nine" - explanation of finale.
The 1968 movie "2001: A Space Odyssey" depicts the evolution of Mankind. An advanced alien race have themselves been through many stages of evolution, moving from organic forms, through biomechanics and finally achieved a state of pure energy. This advanced alien race use black slab crystalline monoliths to investigate worlds all across the cosmos and, if possible, to encourage the development of intelligent life and aid in evolutionary steps. Each monolith is a ratio of exactly 1²:2²:3² (1:4:9), which rules out any naturally occuring phenomena.
The monoliths are located in the movie at pivotal points in the evolution of Mankind. For instance, a monolith is present when chimps first learn to use bones as tools and weapons, another monolith when humans first venture into Space to reach the Moon, and another monolith near planet Jupiter when humans begin trying to colonise the Solar System. So, the movie suggests that advanced aliens are helping Mankind to progress and evolve.
The below assumes you've played through my Left 4 Dead campaign, "One 4 Nine".
In the campaign, the monolith and the tomb are not part of the same thing (the monolith was constructed by the advanced aliens, but the tomb was not.) This is suggested at the finale by the monolith not being physically connected to the tomb in any way, but separated by an area of dirt. The large blocks that comprise the tomb's floor in the finale area are all slanted towards the monolith, suggesting that whoever built the tomb around the monolith did so as a form of awe and worship. pic1
Just inside the entrance to the tomb is a Combine symbol (pic2) from HalfLife 2 - yep, it's a Combine tomb. The Combine built the tomb around the monolith, and at some point the advanced aliens helped the Combine to evolve and explore the cosmos themselves. (In HalfLife2, the Combine subsequently returned to Earth having evolved into alien, synthetic and human elements, and attempted to enslave Mankind).
Some time after the Combine have evolved and left Earth, Mankind evolves from primitive chimps with the aid of the monolith in the movie, and is soon helped to evolve further upon discover of the monolith in the Combine tomb. The tomb had been left abandoned by the Combine a long time ago and has fallen into disrepair, suggested by the fallen blocks and caved-in sand blockages.
The U.S. Army find the sealed tomb exterior. The Army take a long time studying the tomb and trying to open it. They build a Military base around the tomb. Finally they open the tomb by using hydraulic lifts (pic3) to physically lift up some of the tomb's exterior sections, and discover the monolith.
The zombie apocalpyse begins, and four survivors hear rumours of safety and answers to be found at the Military base. They head off towards the Military base only to find it has been overrun by zombies. Graffiti scrawled onto walls provides warnings not to go further, yet the suggestion that the answer can be found if they do press onwards.
Upon eventually finding and touching the monolith, the screen recolours several times and gradually fades to black. This monolith helps the survivors to evolve into something other than their current physical form. The screen recolouring during the finale suggests this transformation taking place. pic4 pic5 pic6 pic7 pic8
The short outro cinematic shows the souls leaving the bodies of the four survivors and entering the monolith (pic10). Their bodies have become zombies, shambling in amongst all the other zombies. Then their souls exit the monolith near Jupiter, and head off into the cosmos (pic11).
The picture after the cinematic shows planet Jupiter and its moon Europa, and the outline of another monolith in the foreground (pic 9.) It also displays text onscreen "The survivors have escaped" - the survivors did indeed escape, but not in the same way as normal Left 4 Dead campaigns (with an escape vehicle.) Rather, the four survivors have evolved into a spiritual form and escaped the Earth, and are now free to travel the cosmos. This leaves their physical bodies on Earth without souls; their physical bodies have become zombies!
So, basically the crux is that the survivor's souls have escaped from the hell on Earth and live on in the cosmos as the next evolution of Mankind, while their soulless bodies become zombies.
2001: A Space Odyssey (movie) 2001: A Space Odyssey (novel) Left 4 Dead Half-Life 2