Bahamut and Zeus hated each other to the point of oblivion. He coveted Bahamut's kingdom and its resources but Bahamut was the strongest enemy he ever fought against and he can't forget his daughter who's capable of demolishing Mount Olympus alone.
Both father and daughter were even formidable when fighting alongside. Long time ago Zeus was able to start a war between them while staying on the sidelines and didn't show himself until the end where he got the chance to strike down the dragon god but he failed miserably.
The dragon god threw them away from his kingdom in a matter of hours. That war led to the decline of strength on the Olympian side. Now he wanted Ryan to grow stronger and fight the Dragon god's daughter Tiamat who also possessed a soul weapon.
Poseidon who is said to be the king of oceans couldn't defeat Tiamat who reigned over all waters. The terror of the ten headed dragon was still deeply engraved on the hearts of Olympians.
