Prior to the Catastrophe
The Izumo Knights was a faction comprised of Ancients, immensely powerful People of the Land. Sometimes, when large-scale quests like the Return of the Goblin King weren't cleared out in time, they would step in to protect the People of the Land.
After the Catastrophe, however, they vanished without a trace, their bases completely empty. Although they were actually all killed by the Genius monsters, none of the Adventurers or People of the Land were aware of that. With the status of the Knights a mystery, the nobles of the Eastal League of Free Cities decided to keep it a secret from the Round Table Alliance representatives when the nobles needed the Adventurers to help fight the monsters that spawned in huge numbers as a result of the aforementioned quest. However, unlike the Izumo Knights, the Adventurers were not necessarily obligated to fight for the nobility, thereby causing much friction between the two sides.