The Demonic Stalker is a very rare glitch that appears in Grand Theft Auto V.
Description[]
On various GTA forums, a number of players reported this very bizare glitch that occurs in Grand Theft Auto V, both in Story and Director modes. Sometimes, a random pedestrian may start to follow and stalk the player with no apparent cause whatsoever. At first, the NPC's behavior will be peaceful but will mimic the player's actions. After some time, the pedestrian will become aggressive and will start to try to kill the player by hand. The pedestrian will be invincible and the player cannot kill him in any way. The aggressive pedestrian will also attack other NPCs that are near the player.
Even if the player leaves the affected area and drives far away, the aggressive pedestrian seems to be in track of the player's location and would continue to pursue them on foot until they kill them. Strangely, this NPC cannot drive any vehicles. If the player restarts the game, the glitch will be gone.
It is unknown what causes the glitch but it is suspected that it may be due to the NPC AI getting confused with the one that a player has on Director Mode (depending on the settings).