Since I dont have any other useful artifacts at the moment, and since I got this from wzib, I just tested it out: The Field Hospital.
* You have to locate it on a territory you already own
* The icon looks exactly like a normal hospital
* It then gives you a discount on armies that you need to conquer any of the surrounding territories
** 12% at rare level as is mine
* You can only have one
field hospital at a time. If you deploy it a second time (after the cooldown), it replaces
the first one