Knowledge of the future grants power over the present. For this reason, Itherael, the archangel of Fate, is a vital member of the Council. He alone possesses the ability to decipher the threads of destiny woven in the celestial Scroll of Fate. His boundless sight grants him perspective others cannot fully comprehend.
I don't know how could work Itherael powers...
At first i imagine something like Architect in Matrix Reloaded: He can simply "calculate" the future by his perception of reality and time. In the begining all creatures of universe are bounded on their role as evil, chaotic, good, etc.so its ability was simply to be descripted as we listen.
With arrival of Nephalem/humans and their free will, possible futures was moltiplexed by infinite possibilities of mortal nature...this could explain why he said "Nephalem's Fate is unwritten" and make Itherael the chaffiest archengel of all Haven, but...
What if its powers work so good to make Itherael controll events by minor little action? No one could suspect him for anything.
Think about current situation:
- Nephalem are a killers easy to manipulate because they trust Tyrael and he trust only their brothers.
- Malthael is dead and Tyrael haven't same old powers. Now there are only three Archangel (like a Trinity if you want) in Heaven. Itherael has a defender and an attacker: Imperious and Auriel. Two angels easier to manipulate then Malthael and Tyrael.
- Diablo universe are more balanced then before: all seven Lords are finally come back in the Hell and Sanctuary must fight them right as Heaven
Some angels don't care about "Good" but only "Law"...and this situation is "fair": Nephalem are stronger then Angels but their world is more unsecured with release of Seven Lords.
In this way Itherael seems an wannabe demiurge that controll peoples and factions with invisible actions, to brings Law and Balance also where no one can see.
What do you think?
PS: Sorry for my bad english.