Unity
Unity was a mystery.
The power that Seeker and Lynd acquired when they saw that vision allowed them to harness the full power of each other's path.
A strange realm connected the two and allowed their respective paths to be passed on to each other.
Lynd was developing the body where he had the foundations of the many Paths and Seeker was able to perfectly copy and replicate the power of each Path.
The pair trained to improve and complete the power and learned how to use each other. But even with numerous attempts to train and develop that power alone, Lynd and Seeker could never perfect or complete the powers on their own.
It was as if they needed each other to summon this power even when the sciences and improvements of their paths already gave them means to use each.
Lynd could never develop a permanent foundation of some Path, and Seeker could only copy other Path if they used Unity.