interventor
One who intervenes; a mediator, especially one designated by a church to reconcile parties and unite them in the choice of officers.
An effort was made, particularly in the Latin church, to correct the disorders of popular elections without taking away the rights of the people. This they did by the agency of an interventor, who was sent among the people to endeavour to unite their votes upon a given person […]
A mine inspector.
Where it comes from
From Latin interventor. Compare French interventeur.
Adapted from the Wiktionary entry interventor (CC BY-SA 4.0).