Legion killed slaves that weren't useful and created more epic slaves. Then they made their slaves have more than one pathway.
This was dangerous work. It required the use of every tool they had on hand in order to ensure success.
People capable of having two pathways are rare in the first place. What's more, these kinds of people with potential can't be determined easily.
The only way Legion could determine how many souls someone has was to enslave them first. This meant that they had to enslave every candidate that came for recruitment.
They didn't have many slots for slaves. So they had to kill the recruits that turned out to be short of six souls.
The scarsity of slots for slaves meant that only candidates with the potential to have two pathways could survive the interview process.
