Martial Coward
"Why don't you croak already, Immortal." A Grade 3 System bellowed in anger, activating its Core Skill as immediately, the energy levels in Immortal began to wane in response.
Grade 3 System—Waning Essence (Level 274)!
This was a System that typically targeted worlds from the cultivation genre, those rich in spiritual energy or other forms of energy suitable for cultivation. And unlike other Systems, Waning Essence gradually weakened that world's foundation, causing the dominant energy in that world to deplete.
The depletion of resources would cause the inhabitants there to fight more to gain the meagre resources, creating a selfish dynamic of resource accumulation by the strong. And it was in these worlds that it harvested Mental Energy.