Essences?

What is the deal with essences? How many are there, and which bosses drop them? You put one of each kind in a cube, with nothing else, press the button, and they turn into something that will respec a character? So far I have one each of destruction, suffering, hatred, and terror. Is there one for each of the 5 act bosses? Does a given boss always drop the same essence? Thanks.|||Token of Absolution

• Twisted Essence of Suffering: Dropped by hell Andariel/Duriel.
• Charged Essence of Hatred: Dropped by hell Mephisto.
• Burning Essence of Terror: Dropped by hell Diablo.
• Festering Essence of Destruction: Dropped by hell Baal.

Transmute the four Essences listed above in the Horadric Cube to create the Token of Absolution. Right-clicking on the Token of Absolution will grant you a Respec.