Another Way (A Date A live fanfic)

A former S rank mercenary was spotted fighting 2 SS class Spirits with only conventional weapons, resulting in death.

The spirits decided to use his body as a vessel to live peacefully. Repairing the damages the 2 had done to the body, unintentionally fixing the soul of the vessel and also unintentionally stopped his ageing

10 years later
The AST, DEM and Ratatoskr have reports of a male spirit seemingly stalking other spirits for the past 10 years. the suspect was usually located easily when a spactial quake was happening, standing at the sidelines or on top of buildings while the AST or DEM was enroute to it. The usual combat report goes along the line of

"We were enroute to the spactial quake location when we located a man standing 100 meters looking at the spactial quake. our squadron decided to ignore it, When we were 25 meters close I took a quick glance at the man when I saw him pull out a silver Desert Eagle on his left hand and shot it towards the spactial quake area. as the bullet was traversing through the air the spirit appeared only to be shot in the chest with the bullet he shot. then another bullet came to her head killing her on the spot. I took another glance at the man and he held a Crimson and Black .44 magnum on his right hand with its barrel smoking. then he disappeared like a flow of the wind"

Officials have observed and decided to name him "Black Plague" because of how he only kills spirits with his Magnum. they gave him a rank of SS class, because of how he only used 2 shots to kill a spirit.