al-Ilaj

As they say, necessity is the mother of invention. al-Ilaj, or the cure, is the concoction, the potion, that is given to all beings, young and old, to strip them of who they are - to make them the same. And that wasn't necessary until Saleh, al Malik, al Ealia was born. Because his mother - the MALIKA mind you - was just a little bit naughty, and he came out looking nothing like the malik, her husband (and clearly not baby Saleh's father. So he needed to be changed, asap. A very skilled and potion master was found, and she made the potion and the amir was changed. And all was well. Until Saleh had his own child. Because she had his genes. And his genes were not human. So he went off to look for that same potion master that changed him (after his mother came clean). She had died many moons ago, but he did find her daughter, who'd practiced under her. And after many months and months of searching through her mother's things, of trial and error, she recreated her mother's work. And that became al-Ilaj - the cure that Saleh promised would bring out salvation, would be the civilians' protection against the plague that ravaged the world outside, and the beasts that roamed the desert and sea.
The truth of al-Ilaj (that is a curse, rather than a cure) is a closely guarded secret, and only a very select few know the truth. However, everyone knows of the existence of it. They were all given it at the time of its first reveal, and every baby born since has been subjected to it also. Pregnant women are given doses of it before giving birth, to ensure the baby comes out human - and nothing else - but the baby is re-injected at birth, to make sure it sticks. All pregnancies must be registered in order for the correct facilities to be organised. Secret pregnancies (which will always be found out) are punishable by imprisonment, or death. This is explained by saying that any child or person without the potion is a danger to the rest of the kingdom. They will be more open to disease, and vulnerable, and perhaps will catch some of the plague in the air.
al-Ilaj requires blood of a being to work. al-Malik takes this blood from the banished Fidayan beings of the Fidayan camp. It also requires magic, and other important herbs and plants, and seeds. It is a complex potion that requires skill and practice, and time to perfect.

Manufacturing

There is a special force whose only job is to traverse the kingdom, or set up shop in different towns and villages, and give out dosages of al-Ilaj.