Not just anyone can possess Amir Gaoul or any key spirit as they possess their own unique set of powers, the also have the ability to choose whose shard caster they belong to, if Amir Gaoul does not choose you then he can cause great harm to the caster's body if it is not the shard caster he has chosen. This is known as "spirit rejection" which is something only key spirits can do. It takes a lot of strength to control a spirit as powerful as Amir Gaoul. In order to control a spirit as strong as Amir Gaoul, that shard caster has to be strong as well in order to be able to control it. If the shard caster is not strong enough, he/she will most likely pass out due to exhaustion or strain on the body.