Mysterious Stranger (Fallout 3)

Mysterious Stranger is a perk in Fallout 3.

Effects
If the last target at the end of a V.A.T.S. sequence has 150 health or less, there is a 10% for the Mysterious Stranger to appear. He will then finish off that target using his unique .44 Magnum.

Additional dialogue options
In the add-on Mothership Zeta, when first arriving aboard Zeta, having the Mysterious Stranger perk opens up a new line of dialogue with Somah, with the option "Maybe, if we're lucky, a friend of mine will show up to help us."