The Maurehazi looks interesting

[Protato]

I haven't read all of Mordenkainen's but the monster I'm most interested in right now is the Maurehazi. I like that it looks reseliant, has some pretty strong attacks with paralysis and stat drain riders, and most all, the fact that it can take the appearance of other creatures before its appearance reverts back, described in a gruesome way as "sloughing off". The artwork is really good too, and when I shared it with people, they seemed to find it creepy. I especially love the claws, they look properly sharp and the red substance could be a paralysis poison, or even rusted-on bloodstains. While talking some liberties with the stats (like the ability to speak Common), I even came up with a good encounter idea for it below (feel free to use, fellow DMs!)

As the group wanders through the ruined city, they spot an elvish man frantically worried about something. He motions over to them, asking for help through bleary eyes as he directs them to a rubble-choked courtyard. However, as they approach, the elf tears his own skin off, revealing he was but a Maurehazi as fiendish reinforcements encircle the adventurers. The lead demon intones, "How gullible you heroes are! This is the last mistake you'll ever make! You'll make fine snacks once we flense the skin from your bones, and drink your marrow like a sweet wine!"

is that close yo how you'd use it? Or do you have a different idea in mind for this fiendish foe's powers?

[M Placeholder]

That's a good idea, but I would have the "elf" basically dig up info on the party and find out what the party knows and how it can use that info, then have the "elf" manipulate the party to fight a group for the fiend, or use the party to further it's goals. Basically string them along, slowly ramping up the fear, confusion and chaos. Then have the "elf" show it's true colours.

[Envyus]

I imagine the Maurezhi would use ghouls and ghasts as backup as they link best with it's abilities.