The divine ability in question is their new Sensory Hijack divine ability. If they use it and Hypnotize to fool White Cloak into thinking that he is safe and that there is nothing to worry about, the soul that escaped would return.
Once the escaped soul returns, they will be able to enslave it together with the other souls using their Puppet divine ability. The only downside to this plan is that it will take time for Sensory Hijack and Hypnotize to fool White Cloak.
During that time, White Cloak can create a lot of problems for them. They prioritized keeping their identity as slave masters a secret, so they weren't willing to waste time converting White Slave into their slave.
Since they couldn't enslave him quickly, Legion decided to give up quickly. In order to prevent problems, Legion gave up on him and decided to kill him quickly.
They controlled Black Axe's body to snap his neck. Then they used Mental Armament to scatter his mind for good measure.
