"The Leth'aar are a group of city-states across several systems united by a common religion, of which a supermajority of the species is devoutly loyal. While embracing a homelife that is ancient and simple, their military, initially similarly primitive, has shown a remarkable, and shockingly fast, adaption to modern technology. Despite their primitive existance, they were able to challenge Republic forces in the Leth'aar-Republic Conflict, also known as the First Contact War amongst their people. These factors, among others, is reason for concern."
-Excerpt from Senate Bureau of Intelligence Report on the Leth'aar
Starting as a hegemony of City-States on the planet Leth'aar, the Leth'aar Hierarchy is a religious group, following a religion based off the theorized Celestials which controls a significant portion of "Wild Space". The Hierarchy is a herarchial meritocracy united in a binding pact, simply known as the Covenant, that forged peace between the city-states of Leth'aar, and allowed for outward expansion, after millenia of continuous warfare between each other that had kept their society in a primative state of existance, around 6,000 BBY. Over time, they conquered Wild Space, using the conquered races as slave labor, and at the time of discovery, had a small empire formed. However, they were not as technologically advanced as the Republic. Despite this, they caught the Republic unaware, and initially led to a devestating war, which quickly turned against the Leth'aar, who were driven back to their homeworld and initial colonies. To avoid destruction, they settled a peace agreement with the Republic.
The government is ruled by a High Council of eighty various leaders, and all are led by a group of three Hierarchs. There are also smaller Councils and Ministries that aid in the governing of the Covenant, with each city-state lead by a Primarch. The people live throughout their daily lives, still in a city-state existance, in a feudal meritocratic society. It is this society that has since unification lead to unparalleled stability, with remarkable advances in technology and medicine despite the lack of outside influence. They are also known for considering other species culturally and militarily inferior. Similarly to other warrior races in the galaxy, they are trained all their lives to be warriors, however unlike similar species and cultures, they consider this role primary in their lives, with all other secondary.