A child is born unable to absorb energy to cultivate, and instead must aquire good Karma to cultivate.

Basic concept for the ability:

Doing good deeds (helping people, forgivness, kindness, smiting evil people, etc.) increaces cultivation

Doing bad deeds (harming people, hatred, attacking good people, etc.) lowers cultivation

Note that forgiveness and hatred are relative. It depends on the reasoning behind an attack. If somebody is jsut being evil, then hatred and revenge is fine. if somebody is doing it to save themselves, or somebody else, then understanding and forgiveness are called for. (The MC is not required to be perfect.)

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No ability to learn techniques. He is only capable of the most basic force application.

If somebody he is not capable of defeating attacks him without provocation, tribulation lightning will strike the attacker.  Provocation includes: attacking somebody, intentionally provoking with words, attacking somebody related. 

Exeption: retaliation is not provocation. if somebody more powerful than him attacks somebody he cares about, he may attack them. if they at all defend or attack back, tribulation lighting will strike. 

He cannot use cultivation resources of any kind. Karma is the only method for him.

Prefered traits of MC (by no means are required): humble, kind, recognizes that he isn't inherently better than others, and knows he's a product of his luckier circumstances (being able to cultivate by doing good things is pretty lucky in my oppinion), loving and caring.