Once they had been normal men. But when the aliens took possession of their bodies, they became immortal, able to read minds; predict and change the future; possessors of weapons more powerful than Earth had ever known. Then their invading masters, with infinite care, made their immortal puppets hate ordinary men. But because they still looked and acted like everyone else, there was no way to tell who they were - until they attacked!
Used availability for A E van Vogt's The Changeling