Spirit Dominion is the ability to have control over the thoughts and actions of spirits. Only two beings are known to possess a form of this power.
As a powerful spirit of the past, Clyde had some control over other spirits. He was able to force two escaped spirits back into his time portal and later arranged for Paige's adoptive parents to appear at the manor and become corporeal.
The Necromancer used it to control Penny Halliwell. Under his dominion, he was able to paralyse her, inflict debilitating pain and influence her actions. His control appeared to subtle but nonetheless powerful, as Penny was fully aware that she was being controlled, yet could not resist him.