The Enemy Weakness?
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"Cutting my head would had probably worked if I was a normal dragon."
"Huh?!"
Frost received a complete shock as he saw Lucifer's head fall into the ground, rolling.
Only for his head to suddenly lift up and speak, as a phantasmal power connected his head with the rest of his body, and quickly reassembled it back.
"An Undead?!" Frost wondered. "But your energy… It does clearly emanates life."
"{Death Embodiment}." Said Lucifer. "The power a Death Dragon has. You'll have to do more than that to actually kill me…"
The power of Lucifer's {Death Embodiment} granted him the ability to literally embody death itself. Giving him the capabilities of an Undead without being one anymore. He was a Death Dragon, between life and death, yet none of the other.
Perhaps freezing him into an ice block could work, or cutting him into meat paste and then burning it all… But decapitation was beyond a lethal hit for someone like him.