Each week, you can swear allegiance to Sabellian or Wrathion in Dragonflight.
While the Black Dragonflight currently has no leader, players can be loyal to either Sabellian, who has returned from Outland, or Wrathion every week. For the week, you will complete quests that only increase your current standing with one of them.
Blizzard also implemented a new UI that displays more information about your current reputations standing with both.