After the release of the second game in the series, Black Plague, it was announced that there would be no more games in the series, due to problems with the previous game's publisher, Lexicon Entertainment, despite the series' original intention of becoming a trilogy.
Six months later, however, Frictional Games released Requiem, an expansion pack focused on story and character closure, and an emphasis on environmental puzzles so strong that for the most part, the game was written off by critics as an attempt to cash in on the outrageous success of Portal. At that time, Frictional also began work on the game's follow-up: Amnesia: The Dark Descent.
Overall, the game was less popular than previous games in the series, in part due to its existence as an expansion, rather than a full-fledged conclusion to the series.
The game begins at the end of Penumbra: Black Plague, with Philip sending the message, "Kill them all," and being knocked unconscious. The player wakes up in a tomb of some kind, and needs to collect items to escape from each chamber and move on to the next. Along the way, Philip starts seeing locations from the previous games, and receives inexplicable communications from Red, his deceased adviser from Penumbra: Overture.
A member of something called "the Shelter" contacts Philip, mentioning that while the Tuurngait do not know that he sent a message to the outside world, the Shelter is fully aware. This turns out to be a deception, however, as the member of the Shelter that contacts Philip is in fact part of the Tuurngait hive-mind.
The game ends in one of two ways. One ending involves Philip joining Red in the incinerator from Overture upon deciding that a regular life was not worth living. As the rest of the expansion seems to have been a figment of Philip's mind, this means he died in the room where Black Plague ended. The other ending concludes with Philip choosing to leave Red to die on his own, returning to the fishing boat that he used to go to the mine in Overture.