It is well known that the Daelkyr led to the downfall of the Dhakaani empire, but this isn't the first, or only, time that a similar event has occurred. Many years ago, another empire was brought low by the daelkyr, but this wasn't Dhakaan, and they weren't goblins. They were a race of gifted psychics, who lived much as those now in Riedra do.
It was from this proud people that Dyrrn the Corruptor sculpted the first illithids, and used them as a weapon to subjugate their own people. They fought the daelkyr with sword and thought, the great leader Gith rallying her forces against the corrupting influence of Xoriat, but there was no victory to be won. There were no Gatekeepers in this world, no knowledge of the primal power that saved Eberron. This land was doomed. Retreat was the only option, and so Gith rallied the wisest of her kind and found a way to open portals into the realms beyond reality.
They regained their strength and evaluated the situation, and here a bitter division split their people. Zerthimon the Wise maintained that the Gith should remain within Kythri, strengthening their will through the endless struggle against chaos. He believed that mental discipline was the ultimate key to victory—that in time, the Gith could impose their will on Xoriat itself, taking the daelkyr’s home just as the Lords of Madness had stolen theirs. But Gith was a warrior, and her followers yearned for battle. They knew they weren’t strong enough to face the daelkyr, but they built their fortresses in the space between spaces and began raiding different layers of reality: pillaging floating towers in Syrania and slaughtering devils on the plains of Shavarath. One day they would find a way to utterly destroy Xoriat; until then, they would hone their skills in conflicts across the planes.
This split remains the case today. Those who followed Zerthimon became the Githzerai, dwelling in monastery vessels in Kythri and gaining power through discipline. Those who split and follow in Gith's wake became the Githyanki, dwelling in enormous fortress-ships, the size of cities, anchored in the Astral Plane. The githyanki can attack any place, on any plane, at any time. They slaughter the devils and angels of Shavarath and steal their weapons, and pillage relics from the vaults of Daanvi (without going through the appropriate channels.)
Where the githzerai are serene and contemplative, the githyanki are passionate and aggressive, two sides of the same coin. Where they are united, however, is their hatred of the daelkyr. Both groups will clash with the minions of Xoriat wherever they encounter them, they have no room for mercy against their ancient enemy.