Anatol might have retreated back into the sword that had imprisoned them for a few eons. But that didn't mean their strength was sealed again. They had just chosen to slightly limit their influence on the world so as not to speed up Millmeria's destruction. They also willingly chose to lend their strength to Ackster by keeping their strength in the sword.
The sword's sharpness, tensile strength, durability, and, most importantly, ability to devour and steal from its targets were leagues beyond what they were before Ackster freed the demon.
The sword was so much more powerful that Ackster felt it cut through the world just by him sprinting toward the Observer. If he wasn't careful, he might destroy Millmeria before the Observer could finish doing it.
Ackster rushed through space using the experience from teleporting inside his Perfect World and flashed behind the Observer. He slashed through the Observer's barrier, ball of cosmic energy, and shoulder.
