User blog comment:B214/Question and Answers/@comment-5185939-20131005195925/@comment-1392308-20131005213216

The real life mythology and the DxD mythology are different. In real life mythology, Midgarosormr was a giant serpent (not a dragon) which was so massive it could wrap around the entire Earth and grasp its tail. It's much much smaller in the DxD mythology and also weaker. It is currently unknown if any of the Mao (save for Ajuka and Sirechz since they are Super Devils) have enough power to kill a god, so if Tiamat can't kill a god, neither can Midgarosormr theoretically.

But not all gods are in the top 10 so it is unknown how Mao and the Five Dragon Kings rank among those not included in the top 10.