After testing out the ability to rescue Conner and the other royal heirs, he had secretly placed the Circle of Dread right beneath the center of the battlefield before the start of the war.
And as the war ravaged the lives of humans and ghosts alike. The sheer amount of negative emotions and vengeful souls that were collected by the circle became so great in quantity that it was terrifying.
So, when he stepped onto the battlefield, it was not Axel's own energy that encompassed the whole battlefield, but a part of the power stored within the Circle of Dread, which made it seem like it was his own erupting demonic energy.
As for the titanic Death Worm, it was actually a special species of creature that only grew on the Ghost Island.
Gareth, by Axel's orders, got multiple hatchling Death Worms from Zoromir, which his subordinate placed beneath the battlefield discreetly.
