feat(tyrant): add arena boss succession

design/concepts/role-and-class-model.md

@@ -8,7 +8,7 @@ description: Defines Tyrant-side classes, Vigilante-side membership, progression
 
 ## Roles and affiliations
 
-A running game has one active or pending Tyrant. It has at most one Vigilante, and a vacant Vigilante role remains open until an eligible player wins the configured arena challenge. The Vigilante may recruit Followers through invitations that require acceptance. The Tyrant may assign the Assassin, Fixer, and Tamer classes after unlocking them.
+A running game has at most one Tyrant and at most one Vigilante. A vacant Tyrant role takes priority in the shared arena; after its winner becomes Tyrant, a vacant Vigilante role may use the same arena. The Vigilante may recruit Followers through invitations that require acceptance. The Tyrant may assign the Assassin, Fixer, and Tamer classes after unlocking them.
 
 Role alignment and Tyrant-side class are independent. A Vigilante or Follower may simultaneously hold one Tyrant-side class, allowing divided loyalties. A player cannot simultaneously be the Tyrant, Vigilante, and/or a Follower, and cannot hold more than one Tyrant-side class. Each Tyrant-side class may be assigned to only one player at a time. Opted-out players are ineligible for every assignment.
 
@@ -37,3 +37,4 @@ Vigilante Follower scaling is capped by default at Strength V and Resistance IV,
 - [Assign unlocked classes](../user-stories/us-004-assign-unlocked-classes.md)
 - [Support the Vigilante and Followers](../user-stories/us-008-support-vigilante-and-followers.md)
 - [Claim the Vigilante role in the arena](../user-stories/us-019-claim-vigilante-in-arena.md)
+- [Claim the Tyrant role in the arena](../user-stories/us-020-claim-tyrant-in-arena.md)