It is tantamount to treating someone as one's extension, an object, or an instrument of gratification. There is physical abuse, verbal abuse, psychological abuse, sexual abuse. The list is long. Narcissists are masters of abusing surreptitiously. You have to actually live with one in order to witness the abuse.
Narcissism is almost entirely about control.
The bulk of narcissistic behaviours can be traced to this panicky reaction to the remote potential for loss of control. The narcissist is a solipsist. He carries the whole universe in his mind. To him, nothing exists except himself. It is terrifying.