The Twilight Covenant was a peace treaty between the Angels and the Faceless that was signed in 0 YSD, bringing an end to the Elder Wars.
When Sar-Elam ascended to Dragonhood, his first act after returning to Ashan was to gather the leaders of the warring factions, asking them to stop the fighting. In order to prevent a mutual genocide, and a disaster similar to the destruction of the Shantiri Empire, he used his new power and authority to impose a peace treaty on them. The two races swore a magical oath that would prevent them from using their magic to harm each other. The pact was enforced by powerful primordial magic that only the Dragon Gods themselves could break.
As the most fanatic and warlike members of both races had been slain or imprisoned in the war, the majority of the survivors gladly accepted the treaty, seeing it as their last chance of survival.
However, the archangel Uriel, still angry at his former foes for the death of his family, looked for a way to strike at them and rekindle the fires of the Elder Wars. He eventually realized that only angels that were alive when the treaty was signed, were bound by it. He therefore planned to rally the Celestials, a new generation of angels, to start a crusade against the Faceless. This began his infamous deception and betrayal in the Holy Falcon Empire and the creation of the Seraphs.