River of Fate

The "River of Fate", as named by Khan, is an anomalous flow of water that supposedly bends reality and takes those that swim in it to the past - as seen in Metro: Last Light. Though the river is reportedly dangerous, this is not the case with Artyom, and even vital in his journey.

Overview
According to Khan, if a man wishes for a second chance at forgiveness, the river will grant him the chance. It will bring you back to the moment before you did what you did. As directed by Khan, Artyom submerges himself in it during the level "Khan", in the hope that he can be forgiven for destroying the Dark Ones.

After trekking there through a small nosalis horde, several supernatural oddities, and a patch of the Darkness, Artyom and Khan arrive at the river. Falling into the water, the two are given a surreal and warped set of visions which all pertains to Artyom's past; mainly the moment he launched the missiles off Ostankino Tower, the moment in "Ashes" in which he chases after the Baby Dark One, and a somewhat upcoming moment involving a crashing train. After seeing the Baby Dark One, Artyom and Khan are released, knowing themselves to have been carried far, and knowing it to be nearby.

As is the nature of metro's phenomena, most of what is thought of the river, save for its effect on the plot, is speculative. It's unknown where the water that feeds it in such large quantities comes from, how long it has been in the metro, and even how long it is. The level of sentience and reverence that Khan attributes to it would suggest it to be some form of intelligent conduit or personification of the metro, fate, or god (the same god Khan himself stated as being atomized in the war). Yet Khan never treats or refers to the river as divine, instead speaking of it as he does of anomalies, with matter-of-fact and instructional detachment - simply spouting exposition as to how the world is not as it once was.

Exhibited Supernatural Oddities
Like the Great Door, the river appears to either have a sort of hallucinogenic or mind-altering effect, or reality-bending properties. As Artyom approaches the river, the laws of the world - particularly of the division between the living and the dead - seem to unravel.
 * In the area leading to the river filled with cobwebs, there is a side-hatch leading to a small room with more webbing. Inside is a skeleton covered in baby spiders. Shaking the skeleton causes the skeleton to twitch into a restful, seemingly bored position.
 * Entering the water causes a silent shock-wave to ripple down from a tunnel, breaking a light, and causing Darkness to spawn.
 * A utility phone rings when Artyom and Khan are near it. Answering it will allow Artyom to hear his mother's voice, and garner him a moral point. Ignoring it will give the Nobody's Home achievement.
 * Further down the water is a metal grate with some skeletons behind it. Approaching it will make them look up and eye Artyom, even turning their heads when Artyom moves. Prior to approaching them, Artyom's pace will suddenly quicken until the skeletons start looking at him.