"Tyrael told me that the winged assassins were once the sages of wisdom. In return for their vow of eternal silence, they were free to roam Malthael's Pools of Wisdom in the High Heavens. Over the millennia, they learned nearly everything about the past, the present, and even part of what would come. But could they have foreseen their dark fates? That I do not know."— Lorath Nahr(src)
In exchange for a vow of silence, the Sages were allowed to roam the Pools of Wisdom. Over the millennia, they learned nearly everything about the past, the present, and even part of what would come. Occasionally a sage reached an exceptional level of knowledge, and were granted the title of Exarch.
After Malthael's disappearance from Heaven, the Sages, along with his other followers, sought him out. Those who found him in Pandemonium never returned, instead casting aside wisdom and gathering in his shadow. They eventually returned to Sanctuary as part of the Reapers, the Sages having been turned into Winged Assassins.