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When Jin-Woo met Legia, he was trapped in the Boss Room of the Dungeon, chained to the wall by several Mana absorbing chains that rapidly stole the strength of the being it touched. The Chains were useless when not connected to the wall.
Legia and Jin-Woo had a conversation regarding the Monarchs and the Rulers, with Legia promising Jin-Woo that he would stand with him against both teams, even linking the 2 of them in a spell that did not let either one lie to one another. Legia also explained that Jin-Woo could not turn him into a Shadow because he was a spiritual being that would turn to dust if he died. Before Jin-Woo was about to free Legia, he found a loophole with Legia's promises, the loophole being that Legia had a 'voice' telling him to kill humans.
Legia swung at Jin-Woo with monstrous strength, even with strength absorbing chains stuck to him for an unknown amount of time, to Jin-Woo's surprise. Jin-Woo proceeded to kill Legia who turned into dust and dropped a round black Magic Core.
After Legia was killed, Magic Beasts in Dungeons no longer dropped Magic Cores and became significantly stronger.