Quantum Theory is a third-person shooter developed by Tecmo, released on the PS3/XBox 360 in 2010. It was originally announced as a PS3 exclusive, but became a multiplatform game. It is notable for being Tecmo's first major project after the departure of Tomonobu Itagaki from the company.
Almost universally derided and ignored upon release for shamelessly ripping numerous elements from Gears of War, it nonetheless shines on its own merits as an overlooked gem, committed to a cohesive idea, theme, and experience. Quite surprisingly, it is largely devoid of the pandering, exploitative, achievement-infested bullshit that characterizes modern blockbuster shooters, and consequently, it didn't sell. The three major factors that distinguish its gameplay from Gears are the large amounts of destructible cover, the frequently shifting architectural elements, and the light platforming/climbing sequences.
The game takes place almost entirely within a heavily fortified tower, gradually being corrupted by a demonic substance known as diablosis. The mysterious warrior Syd embarks on a personal quest to destroy the tower. He finds an unlikely ally in Fileena, who is instead trying to protect the tower from corruption. It is made clear that the events of the story have been cyclically repeating, and Syd's efforts are ultimately futile, part of a grand biological experiment beyond his comprehension. He is obviously based on Guts from the Berserk manga/anime series, and there's a silver-haired Griffith equivalent who achieves a similar “fallen angel” status (just like Sephiroth!). It is also probably not a coincidence that the first published arc of the Berserk manga is set in a monolithic tower.
There is a reasonably full-featured online multiplayer mode with five maps, but it's impossible to find a match anymore, and there's no options for local play.
* Quantums... and diablosis! - SB forum thread.