In order to enslave a legend, not only will the vast quantity of spiritual energy of a legend need to be drained completely, but one will also have to remain alive during the process and not allow the legend to kill them.
The first half of that process is easy for slavers to do because they have the rank 6 Imprison divine ability. With this divine ability, draining and incapacitating a legend is very possible.
All a slaver will need for the Imprison divine ability to work is time and a lot of spiritual energy. But that aside, slavers will also need to somehow remain alive while the divine ability is working.
After all, a legend is not just going to sit back and watch as they are enslaved. The legend is likely to resist, so the life of the slaver will be in danger during the process.
However, despite how difficult this advancement requirement is, many slavers have managed to complete it and become legends.
