The Exarchs are Atlantean mages who ascended to the Supernal Realms by climbing the Celestial Ladder, which they then destroyed. Their ascension granted them god-like power, but the destruction of the Ladder wreaked enormous damage, allowing the Abyss to separate the Supernal Realms from the Fallen World and preventing most mortals from Awakening.
The Exarchs supposedly hold a great deal of sway over the Fallen World, largely through the Seers of the Throne, who believe they are servants of the Exarchs. One of their major goals is blocking access to the Supernal Realms, in order to gain exclusive power over them.