It’s a known fact that Necromongers do not reproduce because they believe that life itself is antagonistic to its natural state and that they need to cleanse this universe from life on their path to another verse that welcomes and cherishes life, the Underverse, their promised land.
Instead of propagating life any further, they are always looking for new converts as soon as they invaded another planet. In the end, all living Necromongers are converts and used to be something else before they did undergo conversion to receive the mark of the Necromonger.
The purification itself is a process to learn how one pain can lessen another. Two painful spikes stabbed into the side of their necks are going to fulfil this lesson and leave the mark.