Find Elder Pannion in his study within the Last Archives.
Elder Gaspar was surprised to see that you have a Shard of the Epoch, much like the one floating in the center of the Council Chambers. He believes that by using the two shards together, you might be able to not only defeat the Void but send you back to your time as well.
After defeating Pannion's corrupted students, you returned to warn Elder Gaspar, but you were too late. Pannion beat you to the Council Chambers and has taken the Shard.