From Halopedia, the Halo wiki
Ulgethon was colonized by the Sangheili as a frontier colony world. A largely barren world, the Sangheili that lived on Ulgethon inhabited a large mountain range, In 938 BCE, the Reformist San'Shyuum arrived on the rocky planet of Ulgethon with their Forerunner Dreadnought. The Dreadnought's Luminary alerted the San'Shyuum of a repository of Forerunner relics hidden in an enormous mountain range on the seemingly uninhabited world. The first pioneer detachment dispatched to the mountains discovered that they were inhabited by the Sangheili. Their first contact with the saurian species proved to be tense, but constructive. The San'Shyuum attempted diplomacy with the Sangheili by sending a delegation to explain that they, too, revered Forerunner technology as holy.
However, the two species had ideological differences regarding the treatment of Forerunner artifacts. When the Sangheili learned of the San'Shyuum's sacrilegious intent to retrieve Ulgethon's sacred relics, they quickly turned against the invaders. Led by an Arbiter, the Sangheili resisted the entreatment of the San'Shyuum and summarily executed the delegation. What followed was the Desecration of Ulgethon, a year long violent conflict that left the planet a charred husk filled with fallen warriors and shattered ships. This event would effectively spark a decades-long war between the two species, known as the War of Beginnings.
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