The Angel Rebellion was a celestial event that occured in 1182 BR, leading directly to the creation of Devils as well as the death of the god Dimensus, starting the Realmshatter.
Azriel Zazael & Krros Zazael, the first angels, are persecuted by the titans for teaching mortals arcane magic, separate from the gods' own. They wish to separate from the gods' divinity, and start an uprising of Angels against the gods. After Azriel's death, The Angel Rebellion ends as Krros rises as the first archdevil through Gunian's magic, burning the roots of the realm tree and piercing the heart of Dimensos, killing the titan at Jöturia. Thus starts the Realmshatter as all the realms are left in a churning mess of chaos.