To get the Desert Eagle pistol in Far Cry 6, you have to complete The Last One To Leave treasure hunt. The location of the Desert Eagle is the Casas del Lodo Residences between Roja Bay and Esperanza.
Go to the Casas del Lodo Residences and begin The Last One To Leave by reading the note on the west side of the electrical substation. You can’t go inside the substation yet, but if you look up on the roof, you’ll see a pole with three cables leading off it. These three cables lead to each of the three switches you need to throw in order to open the door and get to the Desert Eagle. Two of them are inside nearby houses, and one is on a small tower guarded by a few soldiers. There’s a complete guide to finding the switches right here.
The chest inside the Casas del Lodo Residence electrical substation is the location of the Desert Eagle. The Desert Eagle’s huge damage rating makes it one of the best pistols in Far Cry 6, especially when it’s fully equipped with mods. This chest also contains a pair of Techmaster Pants, which improves the amount of damage done by your proximity explosives and C4.