Stalker: Shadow of Chernobyl → Sarcophagus
Before the first right turn, be careful—a sniper with an electromagnetic gun is holed up at the far end of the corridor. When you reach the end, turn left and follow the corridor. Turn left into the passage with the metal mesh and, very carefully, peeking through the hole in the wall, pick off the Monolith stalkers.
After passing through a corridor lined with crates, you'll see a staircase. The stairwells are teeming with enemies. Several more Monolith stalkers await you at the very top, behind a doorway with a warning sign. Beyond this doorway, you'll be able to choose an ending. Turn left and enter the ruined reactor—this will be the false ending. Turn right and you'll find an inconspicuous alcove with a staircase leading up to a creaking emergency light—this will be the true ending.
False ending
You can reach the Monolith shimmering in the alcove by entering the white teleporter rings. All you need to do is carefully navigate the metal structures into the next room, climb the iron sheets onto the concrete reinforcement, and jump down to the Monolith.
And what exactly the hero wishes for depends on many things. For example, on how much money he has in his pockets, on his reputation, and also on whether he accidentally killed the leader of Duty or Freedom.
The true ending
While you're trying to figure out the code to open the door, Monolith stalkers will appear out of nowhere behind you. As soon as the door opens, run inside. You're in a secret laboratory. Go to the "Monolith Control" section.
See also:
Grenade launchers and sights (parameters)