Amnesia: The Bunker

In the year 1916, France and Germany are in the midst of World War I. French soldier Henri Clément heads out into No Man's Land in search of his friend Augustin Lambert, who has gone missing during a patrol. Henri finds Lambert trapped in a large crater. He has Lambert drink from the water he finds inside the crater to revive him, and attempts to carry Lambert to safety. However, they are spotted by the Germans and Henri is knocked unconscious by an artillery shell blast.
Henri wakes up some time later inside a seemingly abandoned bunker with no memory of how he ended up there. He quickly finds a surviving soldier, Albert Boisrond, who informs him that the bunker has been sealed off, and instructs him to find dynamite and a detonator to reopen the exit and begs for Henri to kill him. Boisrond is attacked and killed by a strange creature shortly after. Exploring the bunker further, Henri learns that some unknown Beast had appeared out of nowhere and has been terrorizing the bunker, slaughtering the French garrison one by one. The bunker's cowardly officers fled, caving in the only exit to seal the Beast inside, but trapping the soldiers left behind. With no other choice, Henri searches the bunker complex for dynamite and a detonator. During his search, he comes across more clues left behind by the soldiers and he begins to piece together the events that led to this situation.
Some time ago, the bunker garrison discovered an underground tunnel dating back to Ancient Rome. They began to excavate the tunnel with the intention of using it to stage a surprise attack on German lines, but some of the soldiers began suffering hallucinations and vivid nightmares about ritualistic torture, monsters fighting to the death, and extradimensional architecture. Fearing that the tunnel actually led to Hell, a group of soldiers sabotaged the excavation with dynamite in an attempt to seal the tunnel. The saboteurs were subsequently court-martialed, and the excavation was indefinitely delayed.
Meanwhile, Henri and Lambert gambled with dice to determine which of them would go out on patrol one evening; Henri, still smarting from a series of pranks played by his friend, cheated with a sleight-of-hand trick, causing Lambert to lose. Guilt-stricken when Lambert failed to return, Henri left the bunker in search of his friend. Later, Lambert brought back a wounded and comatose Henri; oddly, Lambert was completely unscathed. It is revealed that the water Lambert drank originated from the tunnel and was part of the ancient rituals; it began to slowly mutate him and drive him insane until he completely transformed into the Beast, stalking the bunker and killing his former comrades.
Henri is eventually able to recover the dynamite and detonator and blasts open the exit. However, instead of finally escaping, Henri accidentally falls deeper into the Roman ruins, where he has one final confrontation with the Beast, where he can either try to fight it (and send it plunging into an underground abyss) or flee.
Henri eventually manages to return to the surface. However, if he failed to kill the Beast, it escapes along with him before running off into No Man's Land. Regardless of the ending, Henri falls into a shell crater filled with the mangled corpses of French soldiers just as a German patrol approaches, leaving his fate uncertain.