Mental energy

This was not because of the sweet taste of the fruit. Instead, when he had chewed the crystal, a pleasant, mysterious but familiar sensation blossomed within him and rushed to his head triggering a slight pain that didn't affect him. Rather, his Mental concentration was boosted!

This was because, the soul crystals contained the soul accumulations of the soul warriors which was absorbed into his own Mental sea once he consumed the crystal. Instantly his concentration standard began climbing at a rate even Xuanyuan found himself astonished of. His mental concentration standard which was only the level of Basic Emperors instantly broke through to Tier 4 Intermediate standard and the improvement won't stop until it reached the pinnacle of Tier 4 intermediate standard.