Essence is what builds up the 9 heavens and 9 hells. Essence comes from an unknown entity which even the most powerful have not deem to have known. Beings utilise essence and circulate it in their body for every action they perform. For circulation there exists essence cores which is existent in living beings soon after they are born. Of course there are some exceptions to this. It is said that the strongest do not possess such cores or to be precise they are powerful entities to circulate essence without the need of a core. Meanwhile some are born as hollows (without a essence core) and they are known to soon perish after they are born.
The strongest are the emperors of heavens and hells following their vanguard. The 9 heavens are named as: Hydra, Carina, Orion, Monoceros, Vela, Mensa, Hydrus, Virgo, and lastly Aries. (Also termed after the constellations over the south-west sky of earth).