These pools are infinitely deep wells of emotion. Those who gaze into them see not their own reflection, but the sum total of emotion that all sentient beings in the universe are experiencing at that given moment. Apart from members of the Angiris Council, many angels frequented the pools to bask in the tranquil radiance that pervaded the domain. Others lingered here for a chance to enter in discourse with Malthael, though few were graced with such an opportunity. Of Malthael himself, it is said that he spent years on end staring into the Chalad'ar here. Deckard Cain believed that the chalice was filled with water from those pools.
When Tyrael entered the realm after taking on the Aspect of Wisdom, he found the area to be cold and desolate, as if it existed just beyond the reach of Heaven's light. A silence that Tyrael almost found painful to endure pervaded the realm, and its crystalline cisterns and founts that had once sung with living light had all run dry. This remained true when the Horadrim entered Heaven through this realm. They found themselves bombarded by all manner of visions, but were able to resist the psychological onslaught.
Winged Assassins were once sages of wisdom who, in return for their vow of eternal silence, were free to roam Pools of Wisdom, learning nearly everything about the past, the present, and even part of what would come.