A veritable relic, Mercy is a pre-War US Army automatic grenade launcher that was used by one of the US Army's counter-insurgency units hunting Chinese Hei-Gui commandos in the American southwest. Presumably during this period, one of its operators spray-painted Hei Gui Bye Bye on the chassis. At an undetermined time, it was obtained by the Brotherhood of Steel and continued to serve as a weapon, eventually ending up in the hands of a Paladin in the Mojave chapter. It was lost when its operator fell while exploring Dead Wind Cavern.
Its higher damage comes at the cost of using 40mm grenades, which are less common and more expensive than the 25mm grenades used by standard grenade machineguns. The larger size of the 40mm grenades also restricts its ammunition capacity to 18 grenades, which is 12 less than the regular grenade machinegun. The 40mm grenade is also heavier than 25mm, further limiting the amount of ammo easily carried in hardcore mode. Like most unique weapons, Mercy cannot be modded.
This weapon looks extremely rusted and used. It also uses 40mm grenades instead of the 25mm grenade like its non-unique version.
Mercy and Vengeance are very alike, despite their names being opposite. They are both heavy weapons, they are both, arguably, some of the greatest guns in the game, and they are both found in the main deathclaw caves.
The writing on the "display" is easily recognizable as part of a Bash script, a very common way of scripting out actions in the Linux and Unix world.