Enslavers are among the most hated creatures in Equestria - by changelings. No-one knows for sure where they came from, but it is rumored they evolved from changelings who were driven out of their hive - loners who were forced to come up with new tactics.  
Without a hive to fall back on, these creatures still realized that a hive would provide them with all they needed.  
An Enslaver usually begins by sneaking into an existing hive, masked by his pheromones, and steals a few eggs. Some will be used directly as nourishment, as they are saturated with love, and some will be kept for later use. As the changelings in the eggs grow, the Enslaver showers them with pheromones to put them under mind control. When they eventually hatch, he sends them out in his stead to collect love. The Enslavers have adapted so well to this lifestyle that theyve lost their ability to transform and fly. Their horns and wings have atrophied, instead giving way to pheromone glands all over their body. They stand a bit taller than the average changeling and dont seem to have any offensive weapons left, instead often running away and abandoning their hives as soon as they are found out. The changelings theyve enslaved wither and die as their bodies cannot survive without the pheromones from the Enslaver.  
Although an Enslavers pheromones are amongst the most powerful found - they can be used to confuse and incapacitate other changelings - a queen is still able to sniff them out, so they have to be very careful whenever they get new eggs. They dont mind the risk much, since their consequent enslaving allows them to live a life of luxury and laziness. In a cruel mockery of the original hive, they are the self-styled kings of their little family.