Evolution.

He chuckled silently as he read through the options for his sin ability. He has been looking forward to them. The options for the UPGRADABLE section are normal for him and expected but the SIN ABILITY section isn't normal even by his abnormal standards but it is to be expected. 

Aeternus knew about the privilege that genius mid-rank demons who were able to meet the requirements for upgrading the quality of their sin energy into the pinnacle rank without actually being a high-rank demon. This requirement is fusing sin energy and mana. He has long met this requirement as a low-rank demon.