I think all of that is basically done in the final fight against Sylvanas. I guess all the Arbiter stuff will be handled in a cinematic and he then eventually steps through the portal and lands atop Icecrown Citadel. In my opinion this is the end of 9.1 and acts as the prolog for Shadowlands‘s climax that’s coming in 9.2 and takes place mainly on Azeroth.