Darkness infiltrated the cells of his body without mercy. It destroyed the cells and regenerated them with slight alterations.
It was not good to undergo a drastic alteration in the cells that had to adjust to the sudden changes in order to prevent the entire human mechanism from facing a complete breakdown.
Understanding this very well, Nial had to endure tremendous pain with every slight alteration within his body. This was also why time seemed to pass so slowly. The darkness continued to seep inside his body, influencing his Heart of Darkness, the mana veins that spread through his entire body, and the Mana Sea that was slowly changing in color.
The change in color was just the beginning because the deepest parts of the Mana Sea were being altered, restricting access of mana. No Mana would be absorbed by the Sea of Darkness once it was completed, but that was not a problem.