Or like I said... The Arbiter. The absolute deadline on introducing a character that will be the main antagonist is the first patch of the expansion.
That's why I think Shadowlands is about role-reversal. The 4 covenants the players join are the 4 aesthetics used for every generic enemies in this game. And the Jailer is like edgelord personified. I think that was done because we're meant to expect he is the main end boss but in fact it's the Arbiter, who looks very angelic but also is just ominous enough to be a big boss fight. Very jrpg final boss esque.