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The Demon King of Death's plan was simple, to reach Yggdrasil and pierce it with a piece of his own body, therefore giving him the power to forcefully take over it.
However, Erebus proved this to be an extremely hard thing to achieve, even more with many of the players manifesting his servant holding his sword out of commission.
His finger alone could never get there, turning into any form wouldn't do, as it would be instantly crushed by Erebus.
He needed someone to wield him, therefore he simply decided to put to good use the knowledge he had stolen from that foolish demonic wizard.
"{Demonic Death Vessel}!"
FLUOSH!
Through the combination of Demon Magic and his own Necromancy, the Demon King of Death created a second body for his loyal dragon general. Their flesh and bones merged together.