In order to get the quest for the path of greatness you've to:
- be level 60 (done)
- do the world mysteries on Isle of Revelation
Look here:
You can obtain pure Astralium from the architect to load the generators.
You can obtain one Astralium for the spying-quest from Holdfast the Honest (You've to do a quest where you've collect 50 gems from draconids)
If you already did that and you got the quest where you've to get 20 Astralium out of Astral Layers, you've to go on Astral Allods yeah. The last Boss will drop it. Or you can exchange one astral key and 200g for 1 Astralium.
Hope that was helpful, if not ask again