Entry to Zerith Mortis

How do I unlock Zerith Mortis zone? I was playing shadowlands when I had to quit to deal with a bout of cancer. I’ve recovered and am cancer free now and I’ve returned to the game. My problem is that I’ve come back to a character with a couple of different progression chains but I can’t remember which I need to work on to progress to unlock ZM> could you nudge my memory please? Which quest chain do I need to work on to unlock ZM?