shivermaw is up
and i got the artifact on both 120 warrior 
What’s so special about Shivermaw?
Shivermaw is a dungeon boss in the Legion version of Violet Hold. On heroic, she can drop boots with the mog appearance that was missing from the ICC-25H Warrior tier set. The thing is, that’s only on heroic, and AFAIK which bosses you get on heroic is random. So her being up for week’s mythic Violet Hold rotation doesn’t help any.
Unless Martius meant something else, of course, because I don’t believe Shivermaw is related to any of the Warrior artifact hidden appearances. The Fury Warrior one requires drops from Skovald, Shar’thos, and Nithogg. So maybe he meant one of those instead.
I don’t see the point in trying to finish that set because it has no belt.
Bah, who’d want to use the ICC belts anyway. They’re giant ugly things. I did farm up those boots, and I’m using them and part of the ICC set with my current mog if you want to see what you can do with them.
I’m not unhappy with them, and honestly mog farming feels like the most productive thing to do at this point in the expansion. I’ve hit all my progression goals and farming increasingly small gear upgrades that will all be wiped away before the next raid tier feels pointless.