RPGMemes

It's a thing about Law, specifically the realm of Law, Mechanus, who's power I would guess they carry with them.

One of the (I think optional in 5e) realm rules for Mechanus is you do not make attack rolls. Instead, you treat any roll as a 10 and add your stat and proficiency as the realm removes the idea of random chance and focuses on clockwork regularity. Even damage die default to their average, which makes combat a lot more straight forward but also means you can come up on something you literally cannot hurt.

There's a handy common level magic item in Xanathar's Guide, called the Clockwork Amulet, which let's you skip an attack roll and just take it as a ten, and do so once per day (No attunement, resets at sunrise). It's a good thing in certain situations

A well roleplayed Marut is great. As a CR 25 construct, it's going to be enforcing a universal law, such as attempting to stop a world ending threat.

It's primary aim is normally to planeshift it's quarry to court, rather than kill them, and may even prefer not to kill those who come between that goal.

Here's The Monsters Know What They're Doing's blog post on them. It really telegraphs how helpless a targeted creature is to them.

Good news is they really only exist to punish players who pissed off the DM, and would therefore hit anyways.