Loaded Gun (5e Equipment)
From D&D Wiki
Weapon (Firearm), Legendary
Who knows how it was made, but it’s cool! Similar looking to a Desert Eagle, it has but one bullet loaded inside.
Firing To use the loaded gun, pick a target within sight during or outside of combat, the target instantly dies if it is alive or is destroyed if it is an object, and a charge is expended. This item has one charge.
Discovery It can be found falling from the sky on top of a powerful warriors head(whoever the warrior may be, for simplicity, let’s say they’re an elite dragon hunter in which the party hasn’t heard of), knocking the warrior unconscious. The warrior is wearing +3 mithral plate armor, and wields a +3 adamantine slivered greatsword.
Any other form of discovery is acceptable and quite possibly encouraged, but this has been deemed comical and a good place to start for GMs.