10) Malachor V/Trayus Academy
Knights of the Old Republic II:
The Sith Lords
"There is a place in the galaxy where the dark side of the Force runs strong. It is something of the Sith, but it was fueled by war. It corrupts all that walks on its surface—drowns them in the power of the dark side. It corrupts all life, and it feeds on death."
Throughout the KOTOR series you are told about the Mandalorian Wars and how the Jedi Knight Revan brought them down, but you never find out exactly what happened until the events of the second game. Revan planned to lure the Mandalorians to the planet Malachor V (which was taboo for them) for one final skirmish; however, he secretly had a weapon constructed that would draw all the ships in orbit and smash them into the planet's surface. This destroyed not only the Mandalorian and Republic fleets, but completely devastated Malachor itself.
When Revan reemerged as a Dark Lord of the Sith, he returned to Malachor and, sensing the planet's Dark side presence, established an Academy for the training of new Dark Jedi and Sith acolytes. After Revan's redemption, his former Jedi Master Kreia (having fallen herself and taken the name Darth Traya) and her two apprentices took over the academy, until they betrayed and exiled her. At the climax of the game, you crash land on the broken surface of the planet and set out to find the Academy and stop Traya.
From the moment I stepped foot on Malachor, I felt an immediate sense of dread. The music is pretty chilling, the only enemies on the surface are mutated beasts, and there are many wrecked ships left over from the catastrophe nearly two decades prior. You are forced to explore by yourself, and (if you play as a good guy) one of your teammates also wanders off and is confronted by her nemesis, a deceased Wookie that Kreia resurrected. Once you finally reach the Trayus Academy, things don't get any better. You are stalked by the Academy's inhabitants, and ultimately you encounter Darth Sion, the zombie-like Sith Lord that has been hunting you the entire game.
Ultimately you find yourself at the center of the Academy and face off against Kreia, and the weapon is activated for the last time, destroying Malachor for good.