Aura.
When knights reach the level of 3-star, they learn to manipulate the mana in their hearts to amplify their weapons or bodies, utilizing the power of magic. The concept of acquiring this ability may be simple, but mastering it is a monumental challenge. It demands precise control over mana circulation and manipulation, making it an arduous feat even for the most skilled knights.
Although three stars serve as the base level qualification to learn aura, there are some individuals who struggle to grasp this skill, even when they have surpassed the 3-star knight level. Unfortunately, I was one of those individuals in my past life. Despite reaching the 4-star level due to my physical abilities, my messed-up body and mana veins hindered me from properly circulating and manipulating mana, rendering me incapable of wielding aura.