Astaroth, also known as 'The Black Giant,' was created by Kupaetku by order of the God of destruction, Ares. Ares was seeking what mortal fighters were, Soul Edge, and his worshiper, Kunpaetku, happily obliged. Astaroth set out to find Soul Edge as ordered, but found it to broken, half of it's power gone from the events that took place in Soul Blade.
Soul Calibur - Astaroth encountered Nightmare, who agreed to work with him, gathering souls by killing many victims in order to restore Soul Edge (victims such as Maxi's crew). Astaroth knew that Ares would not be pleased to receive the broken sword, so he decided that when it's finally back at full power, he'll get rid of Nightmare and deliver the sword to his master. After three years, it was fully restored, but before Astaroth could get his hands on it, a pirate seeking revenge battled and killed him.
Soul Calibur II - After four years, Ares resurrected Astaroth, still seeking the sword. Astaroth was different, however, as his body absorbed pieces of Soul Edge. He became much stronger than before, and so continued his quest to find the sword. During this time, Kunpaetku decided to put a curse on Astaroth, because he seemed more aggressive than usual, not listening to his orders. This was because the agent of death and servant of Ares, Kerr, was inside of Astaorth.
Soul Calibur III - Kunpaetku placed the curse to bring Astaroth under his control again, but it had no effect. Astaroth decided that he had no use for Kunpaetku or his cult anymore, and went to the shrine to kill him. Kunpaetku managed to get away, telling Astaroth that he was based off of a human known as 'The White Giant.' Astaroth believed that this 'White Giant' had to be strong like he was, and forgetting his true purpose to find the Soul Edge, went searching for this giant, who was Rock, so that he could kill him and prove who's stronger.
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