The ancient history of the Star Realm passed down the story of the Legendary Mage who became the Godslayer—it isn't a fabrication. Although these Legendary Mages achieved their godslaying feats in the Material Plane, where the Divine Spirits were suppressed with perhaps only a fraction of their power remaining, godslaying is godslaying.
The insight, experience, and power of Divine Spirits, compared to ordinary Legendary, are simply in different dimensions. Those Legendary Mages who succeed in godslaying often rely on digging into their potential to its limit and developing incredibly powerful Forbidden Magic to achieve such feats.
Of course, not only the Mage profession can accomplish the feat of godslaying; some Abnormal Dragons with extraordinary talents in the Giant Dragon Race, terrifying creatures wandering the Star Realm, and ancient forbidden Epic Professionals all exist who transcend their tiers to achieve godslaying feats.
