Bethians were a clan of the Argenta who originated from the outer worlds of their star system and created the city of Hebeth on the surface of Mars. The city served as a transport hub through the use of a system of teleportation called "Slipgates," but was lost to the Sentinels and swallowed up by the Martian surface, becoming a lost city.
The Doom Slayer ultimately exposed the ruins of the lost city when he fired the BFG-10000 at the surface of Mars, as Hebeth had one of the only remaining portals that could take him to Sentinel Prime.