Let's start with: 'What is SST?' This is called Supernatural Special Team. Team, that deals with special cases associated with supernatural events and crimes. Of course, they work perfectly legal as the Police Department. Something likes demon hunters to the police. The president of the country ordered the creation of this team. The cops did not like SST. They call them with different names, most commonly used: 'a bunch of crazy lunatics with guns.'
Why the existence of the Unit is necessary? Well, the reason is simple. The supernatural beings (those who should not exist or so-called 'fairy creatures') decided to not hide anymore. Exactly for that they made them part of society. And as you've probably guessed, not everyone agreed with this. However, who wants to be surrounded by murderers or potential ones? For this the department was created, to deal with them.
Originally the Team had only human members. But once it became clear that people are not sufficient, the chief took quite a difficult decision. They recruited several… non-human beings. While the witches and wizards (in all its varieties, including necromancers) are considered midrange. Initially the voltage was pretty high, but other times this changed. People and the others began to live in a relatively harmonious relationship.
How I know all this? Let's say that I have experience with this… Organization. But it has yet to understand.