https://ck2.paradoxwikis.com/Adventurers
If nothing unexpected happens while he was preparing for his adventure, the character leaves their court to lead an army from the same location. They gain a duchy-level "Host" title and several courtiers to use as
commanders, as well as the Adventurer
trait. He create an army whose size scales with the strength of the targeted realm, making them dangerous even to a large realm. However, the total number of troops in an adventurer's army will not exceed 30,000. These troops are considered
event troops, making them immune to any form of
attrition, but also impeding their
reinforcement. The war follows the same mechanisms as any other
claim war, except that the defender won't be able to get any occupation warscore, as the attacker does not possess any land. As such, just like in a
rebellion, the only hope for the defender to win the war is to eradicate the adventurer's army and to ensure none of his territory is occupied.