Deep darkness. That was what Onyx was sensing. He had been able to learn both light and dark mana. That was a blessing of his species as an abyssal serpent. Yet, he had not felt the true depths of dark mana that Morant had become privy to.
However, that did not limit the help that he was giving to Mordant. He was there not to control the manas, but to act as a counterbalance so that they would not be running themselves out of bounds.
What this meant, was that onyx would be the one that added the slightest amount of light mana around. This sounded counterintuitive to those that could not sense the changes in light and darkness manas when they came in to contact with one another. However, when the dark mana becoming denser here was pushed by light mana,
Because the light mana would push the dark mana, it would ensure that the dark mana was being guided in to the rune as they were completed. But he also had to focus on more.