Nyxara had been at the right place at the right time, and she had benefited from her brother's demise. Knowing that no Primordial in his right mind would give up their powers even if they knew it would lead to their ultimate victory, if they were going to die, then they would not want any other person to benefit from their death, not unless they hated someone well enough to forget their innate selfishness.
All of the Ancient Primordials were planning ways to betray their siblings when the time was right, and for them, that time would come when all of their enemies were dead, and ultimate power was in their grasp.
However, they had made a fatal error, and that was that they had underestimated how much power Nyxara could gain from the simple fusion between Soul and Light.
Nyxara craved more of that power, and she would go to any length to acquire it, even if it meant betraying her entire family.
