Invitationals (or Invitation Duels) are started by inviting players at the Invitation Board after reaching the required Player Level and using a 
Match Invitation Letter. Diluc, Kaeya, Sucrose, and Fischl do not require a Match Invitation Letter.
Completing the World Quest A Guest From Liyue allows characters from anywhere in Teyvat to be invited, provided the Player Level is high enough. If A Guest From Liyue is not completed, Inazuma characters cannot be invited until Radiant Sakura in Raiden Shogun's Story Quest, Imperatrix Umbrosa Chapter: Act II - Transient Dreams is complete.
