It is narcissism. They have no self, every aspect of them is a facade.

They meet someone with traits they want, usually someone who is kind, compassionate, forgiving etc (and possibly codependent) and they put them on a pedestal. They love bomb them and treat them like the most amazing person in the world. Then they realise the reality doesn’t match the idealised person.

They require your admiration and emotions (narcissistic supply) but when that runs out they discard you. They have usually already found someone else, another favourite employee or lover, to supply them.

With a narcissistic boss, the only survival technique is to be the most boring person on the planet to them. Never show emotions or talk about your personal life.