Posted April 18, 2018
Not on GOG since the rightsholder loves control and DRM, but:
Diablo 1. The final boss is "The Dark Lord" (of Terror). When you defeat him, a post-game movie shows the protagonist remove a cursed gem from The Dark Lord's head, causing The Dark Lord to shrink and become the possessed prince you've been seeking the whole game. The protagonist just killed the one human he specifically promised to find and protect. It gets worse. In an effort to contain the malevolent spirit, the protagonist then jams the cursed gem into his own head, hoping that he (or, if you played the Rogue, she - but you don't have a separate movie for that) is strong enough to contain the demon. The sequel says he (or she) failed, and The Dark Lord possessed the protagonist of Diablo 1.
Diablo 1. The final boss is "The Dark Lord" (of Terror). When you defeat him, a post-game movie shows the protagonist remove a cursed gem from The Dark Lord's head, causing The Dark Lord to shrink and become the possessed prince you've been seeking the whole game. The protagonist just killed the one human he specifically promised to find and protect. It gets worse. In an effort to contain the malevolent spirit, the protagonist then jams the cursed gem into his own head, hoping that he (or, if you played the Rogue, she - but you don't have a separate movie for that) is strong enough to contain the demon. The sequel says he (or she) failed, and The Dark Lord possessed the protagonist of Diablo 1.