Beat the final boss - Mithrix on the hardest "Monsoon" difficulty.
Or, the funnier method, also on the "Monsoon" difficulty. Get into the "loop" (you need to change the alignment by clicking on the teleporter's external part on the 5th stage). You will return to the first stage (but now it's 6th stage).
Then you should go into the Celestial Portal after the 8th stage teleporter event. There you need to obliterate at the obelisk. Achievement completed!