Mordenkainen Nightwalker

[Daithi]

At 14th level Necromancers can Command Undead. An undead creature within 60' that fails a CHA saving throw becomes friendly to you and follows your commands. If the creature has an INT of 8 or higher it gets advantage on its saving throw, and if it has an INT of 12 or higher it can repeat its saving throw every hour until it breaks free. Well...

The Nightwalker has a CR of 20, but its INT is only 6 and its CHA is only 8. I think I want one of those as a permanent "friend".

However, there is in line in the description of the nightwalker that says, "nightwalkers exist to make life extinct and never to serve living things." It doesn't say that can't be forced to serve by a spell, but of course DMs could rule otherwise.

[hamishspence]

There's also a line in the description of the gibbering mouther saying that they are some of the most depraved and vile creatures ever to be created by magic - with the listed alignment in their statblock being Neutral.

Or a book talking about Tempus's "chaotic nature" but giving him a listed alignment of Neutral.

Sometimes there's fluff to rules mismatches.

[Blackbando]

I have two thoughts on this as to why it'd work:

1. It exists never to serve things, yes, but it only never does willingly. Which brings me onto 2:

2. Command Undead rips away the undead's free will/self-control (or, in the case of intelligent undead, most of it), making it completely under your control regardless of what the original creator or it desires. It's basically a potentially permanent dominate monster, in a sense; you aren't asking the undead for permission to control it, you just are going to control it (or at least try).