Dominions 5 Wiki

A raid is a move a commander may perform only if they have the Pillager ability. If all units under his command also have this ability, they can join the raid. A raiding party targets any adjacent province, where it kills population, causes unrest, and gains gold. It is essentially the same as the Pillage order, except it is performed from an adjacent province. The raiding force returns to its starting province after the raid is complete. Raiders may be discovered by province defense or patrollers. A limited combat is initiated if the raiders are caught. In this case, a battle will commence with province defense + patrollers versus the raiders. Winning the battle will not conquer the province. Additionally, a battle prevents any pillaging that turn, regardless of outcome.

Raiding has extremely reduced effectiveness (less than %10) on independent-owned provinces.

Multiple units Raid-commanding in the same turn will be detected individually and fight patrollers individually. However, the units on Raid-command will also reduce the ability of patrollers to catch additional Raiders, so it is unusual for patroller to catch more than a few units if multiple units on Raid-command are engaging in the same turn. Damage done by subsequent Raiders is also gradually reduced. So additional units raiding the same province cause reduced damage, but the risk of being caught/fought is also reduced.

Raid Command can be combined with Sailing to use Raid-command on non-adjacent coastal provinces. (no test has been done on whether or not the army can retreat to the original province if they are caught, though)

You can read more on Raid here