The rebel army can form when Public Order reaches -100 in a Tomb Kings Province and will begin to raid or attempt to attack the nearest settlement. They do not appear in the Diplomacy screen and cannot be negotiated with.
Tomb Kings Rebels can become very powerful, especially in the early game. When your armies are mostly skeleton warriors and a few Nehekhara warriors, the rebel army can recruit from the full roster. Meaning it is possible for them to get a Hierotitan or a Khemrian Warsphinx. Units which will take hundreds of skeletons to take down. As such rebellions in the early to mid Tomb Kings games need to be avoided at all costs or crushed immediately.