Feluxers

Feluxers, also known as the Sinners are the people who "prayed to the God Felux" to stigmatized them and gain the ability of "looting" a Rune User upon killing them. 

They are from the City of Misfortune. This has been told a few times already but the City of Misfortune is the city in the Gantlet Desert, left by an unknown group.

The first residents there had speculated that it must be a city of ancient Rune Users who worshipped the Hell God of Souls, Felux, and built a cult. There are a few remnants that support this theory, their only clues.

Other than that, there are no clues in the world of Gantlet. They could only speculate from the natural clues and how the world was built, the monsters, and the system but no concrete clues.

As if they were intentionally removed...

There was never a conclusion. But they never needed one so they never desperately searched for them. They were busy fighting each other using strength and wits.