Knights of Sumeria is an ancient and mysterious group of people sworn to protect Earth from the Deadites. They are introduced in Ash vs Evil Dead Season Three. Dalton, who is a member, briefly mentions them a few times.
Almost nothing is known about this group. They have been around for centuries, probably battling evil and all sorts of demons throughout the decades. However, KOS suffered a devastating defeat in their mortal battle against Ruby, back then known as Mulier Daemonium de Igne. For the next centuries KOS would slowly rebuild themselves and prepare for low-profile search for Ash Williams, the foretold Prophesied One.
During Ash vs Evil Dead season three, Dalton joins Ash's posse and helps them in their quest. He tells them that he is a knight and warrior and believes that Ash is the prophesied one. The person who'll protect humanity from the deadites. but Dalton ends up falling victim to deadites and commits suicide. Fortunately some time before Dalton died he requested for reinforcement and more KOS agents arrived to help Ash and his gang, only to be decimated in their first real fight later.
Zoe survived the encounter, only to be abducted by Kaya and Ruby later one. They used her as a vessel and leached her blood and skinned her so that they can write a new page into the Necronomicon. When both her captors were busy, Zoe tried to escape, but met her end at hands of Kaya. Before her death Zoe recited one last time that the true savior will avenge her and kill them.
It seemed that with Zoe and Kaya gone, all known members of the Knights of Sumeria are deceased or banished back to hell, but after Ash fought the Kandarian Demon and defeated it, agents belonging to KOS wearing full MOPP gears showed up and carried his unconscious body to safety. Ash was then put into a cryo-sarcophagus so that he could regenerate and heal up after the fight.
When Ash wakes up, he finds himself in the future where seemingly no one survived. Lexx shows up and introduces herself as an another member of the KOS. She, like her precusors, sworns to protect and serve Ash. Lexx also tells Ash that Dark Ones are still present and are still a threat to humanity.