Here is the full list of all 23 Empyrean achievements.
Shoot down at least five enemy aeros.
The Air Guard captain, the Quon Bexi spy, the revolutionary, and the machine-girl all survive to the end of the game.
Maintain an impenetrable cover identity.
Only crash once.
End the game with the Empyrean in your possession.
Destroy at least three Deep Tech creatures.
Reach 75 Empyrean at any time.
Unify the Combines and the revolutionaries.
Fight a sword duel on the wing of an aero.
End the game with Revolutionary Character 80 or higher.
End the game with Combine Character 80 or higher.
End the game with Actorius Technology 80 or higher.
End the game with Actorius Grandeur 80 or higher.
Romance the Air Guard captain.
Romance the Quon Bexi spy.
Romance the revolutionary.
Romance the machine-girl.
Betray Wesh.
Double-cross your father after betraying Wesh.
Fly the Gossamer.
Reprogram an automaton.
Find the Iridium Garden.
Identify Lectini as the Quon Bexi spy.