Mysterious Stranger/Fallout 3

Mysterious Stranger is a perk in Fallout 3.

Effect
The perk activates a 10% chance for the Stranger to appear whenever they are using V.A.T.S. When the Mysterious ally appears in all his trenchcoat and fedora glory, he will empty his .44 Magnum revolver into the target, dealing 18 002 of damage (9 001 base + 9 001 critical damage at 100% multiplier) per shot. One shot is enough to kill any enemy, but the Stranger will always fire all six shots for a potential 108 012 points of damage. The game keeps track of the visits and will randomly display their number on the loading screens and under the Stats/General tab in the Pip-Boy 3000.

Having the perk also unlocks an additional dialogue option with Somah at the start of Mothership Zeta.

Note that the Lawbringer, Contract Killer, and Grim Reaper's Sprint will not activate if the Mysterious Stranger appears, as they're dependent on the player character dealing the killing blow. Furthermore, as the Stranger dies not use a script, but is an actual NPC spawned by the game with a functioning weapon, it's possible for him to spawn stuck in a wall, submerged in water, or otherwise placed in a position where all six shots will miss or he will kill the player (if he spawns in a way that places the player in his line of fire). Player weapons that deal area of effect damage can affect the Stranger, but he cannot be killed.

Bugs

 * The spawning can also bug out in a number of ways, with the game correctly playing the jingle, but the Stranger not appearing, or him appearing, but not shooting the target (if the player has already killed it with the VATS attack). The Magnum can also jam, forcing the Stranger to reload, rather than fire. The VATS sequence can also stay active for several seconds, before resuming, during which enemies have a window of opportunity for an attack.

Behind the scenes

 * The Mysterious Stranger perk was originally designed by Scott Bennie.
 * The Mysterious Stranger in Fallout 3 and Fallout: New Vegas wears a fedora and overcoat, like the protagonists of 1950s film noir.