"Imagine this...
Necromancers are the most hated yet most wanted people. They are hated for using the dead. Yet it is this very skill for which they are desired. Kings would do anything to get a necromancer to help them in time of war. They would give them wealth, power, fame, anything you can think of. At least that was how it was. It all changed when people found out that necromancers do not have to be the one controlling the dead. They can transfer control to someone else if their blood is drunk when they are summoning the souls of the dead. Once this news spread out, it was a catastrophe for the necromancers. They were captured and sold like objects. Being physically weak, they submitted easily.
It was a sad reality. The once-proud necromancer became slaves to those who they looked down on. Eventually, their already low numbers started decreasing, dying of blood loss. Until only a few were left. And those few having learned what it would mean if their secret was revealed escaped their masters using the last of their energy and hid away.
As time passes those searching for them gave up. And the world forgot their existence. They say that those necromancers are still out there in hiding waiting. Waiting until even the myths about them are lost to time. And when humanity lets it's guard down, their descendants will attack.
They shall once more be feared. And that day they attract is this very one."
The screams start as the corpses crawl out of the ground.
"Don't worry. We will not kill you. We are just here to take what you took from us."
"Our freedom."