To start this mission, talk to Mary May in the Spread Eagle Bar in Fall’s End. She will ask you to retrieve the custom semi called The Widowmaker which the cultists stole.
Next, head to the Grain Elevator and take out the peggies hanging around the area. This is the same location as the Prepper Stash “Vespiary” so if you already completed it, you just need to backtrack where the stash is located then use the zipline to reach the roof of the garage where the Widowmaker is kept. Drop inside then open the garage door. Hop inside the Widowmaker and drive it back to Fall’s End.
On your way back, you’ll encounter cultist vehicles attempting to intercept you. The Widowmaker is armed with twin vulcan cannons that can easily rip targets to shreds. You’re practically unstoppable so don’t hesitate ramming enemy vehicles and breaking through roadblocks.
Keep driving along the main road and following the mission markers until you get the prompt to deliver the semi back to Fall’s End. After parking it beside the bar, leave the vehicle and talk to Mary May to complete the mission. The Widowmaker is now permanently unlocked in shops and can be spawned from any garage.