An independent person or body officially appointed to settle a dispute:

The one word substitution is Arbitrator.

Arbiter : a person who settles a dispute or has ultimate authority in a matter.
Mediator : a person who attempts to make people involved in a conflict come to an agreement; a go-between.
Agent provocateur : a person employed to induce others to break the law so that they can be convicted.
Arbitrator : an independent person or body officially appointed to settle a dispute.