The Golden Egg and the Black Egg merged during the time Klaus was absorbing the True Demon Core to boost his cultivation.
Right when they merged, Klaus sensed something incredibly powerful fuse with his soul.
He wasn't awakened back when the Golden Core merged with him and extracted the System given to him by the heavens when he awakened his Class.
Back then, during the extraction process, both the senior and that sneaky Black Egg hidden well within Klaus's soul all this while worked together to extract that System, and the Black Egg absorbed it.
But it didn't end there. After consuming the System the heavens gave him, it went ahead and tried to devour the Golden Core.
However, it quickly realized that it was impossible, so the senior proposed a truce in which the two should merge and become one.
Back then, the senior merely wanted to ensure there were no two powerful entities clashing inside Klaus's soul, so he proposed that idea.
