Silent Death

Silent Death is a Fallout, Fallout 2 and Fallout Tactics Perk.

While Sneaking, if you hit a critter in the back, you will cause double damage using a HtH attack. Silent Death is that kind of Perk.
 * Note: A weaker equivalent of this Perk called "Backstab" appears in Lionheart. It has also been carried over to Fallout 3 in the form of sneak attack criticals, which does not require any perk.