Tomb Kings Patrols.
Tomb Kings now have their own version of Province Patrols. Tomb king patrols work different from the normal default patrols. When a Tomb King faction owns an entire province the province patrol of that province will activate. This patrol is hidden beneath the sands in the province so can't be seen or attacked and will ambush any enemy army.

Whenever an enemy army stops within a region with an active province patrol there will be a chance that the province patrol will ambush the enemy force.
The chance for the patrol to emerge and ambush is determined by the amount of turns the enemy army has been inside the province and the stance the enemy army is in.
Multiple armies increase the chances of a patrol ambushing.

When you're playing as the tomb kings you can see the % chance for your province patrols to ambush in the effects box of your province.
This chance is only displayed whenever an enemy army is inside your owned province and a province patrol is active.
Whenever you're in an enemy tomb kings region and the province patrols ambush chance is over 50% a dilemma will be triggered for the player informing that a province patrol is coming.

Ambushing while going through a province will give you the lowest chance to be ambushed
raiding or marching while going through a province will give you the highest chance to be ambushed
If a tomb kings patrol tries to ambush an army that is encamped, a normal battle will trigger instead of an ambush battle.
Travelling in the underway stance while going through the province will let armies remain undetected by a province patrol.

After ambushing the patrol will always need to replenish.
If the patrol won the battle the province patrol will replenish for 2 turns.
If the patrol lost the battle the province patrol will replenish for 5 turns.

Each playable tomb king faction has their own unit pool in theme of their faction.