Maruts are a species of extraplanar golem-like monster that have appeared in every edition of Dungeons & Dragons to date. Their lore has changed a lot between editions, but the core idea of a single-minded, automaton-like pseudo-angelic being has remained consistent throughout all editions.
Maruts made their first appearance in Advanced Dungeons & Dragons 1st edition, where they are stated and detailed in passing in the original Manual of the Planes splatbook. This information is extremely bare-bones; maruts are presented as golem-like servitors of the god Rudra, having the form of "great, red-eyed, unliving giants carved from polished black stone and dressed in gold armor with wide plats on the shoulders and armbnds".
They would subsequently be updated to 2nd edition in the Outer Planes edition of the Monstrous Compendium Appendix for Planescape. Although described in more detail here, the base concept is still quite simple; maruts are literally angel-golems, combining divine power and allegiance to a deity with the single-minded purposefulness of a golem. They exist only to serve their god's will; nothing more. Though still described as having been first created by Rudra, they have also spread into the hands of other gods, mostly as payments from Rudra.
In 3rd edition, Maruts were reinvented, most prominently by gaining an extended family... who haven't appeared in any edition since. The reason why the Inevitables came to be is unknown, but we know that Modrons were pushed out of the spotlight in this edition due to being regarded as "too goofy". It's possible the aim was to minimize the outcry for modrons returning by filling the niche of "golemesque mechanical outsider" with Inevitables.
Anyway, in this edition, Maruts lost their connection to the gods; instead, they were entities of Cosmic Law, entrusted with the task of preserving one of the Great Laws of Reality in order to keep the multiverse from collapsing in on itself. Specifically, maruts in 3rd edition are custodians of the Law of Life & Death - "Thou shalt not cheat death". They go after any being who seeks to become free of death's grip, from powerful wizards to ancient liches and vampires, as well as those who have a habit of being resurrected (like many adventurers) and necromancers (due to their creating undead in abundance), not stopping until they have destroyed their target or been destroyed. They gained exaggerated Roman Centurion-style helmets in this edition, and are noted as being the least-personable and most mechanically minded of their kinsfolk.
In 4th edition, maruts changed yet again, losing their Inevitable kindred. Ironically, they gained a lot of personality in this edition, being described as thinking, philosophical beings with individual minds and personalities. Maruts of the World Axis were created by the gods to be the ultimate adjudicators; completely impartial judges who would thusly be wise enough to act as mediators even the gods could not argue with.
World Axis maruts exist to preserve, honor and enforce contracts, a role that often leads to them acting as mercenaries, guards, police and other forms of enforcement. In truth, the maruts have their own great goal; to weave a web of contracts, favors and obligations that they can use to ultimately force the multiverse into adopting a more lawful, orderly format. Ironically, in creating the ultimate judges and mediators, the gods have created a race of would-be tyrants, compelled to seize ultimate power over the multiverse so they can rule it with an iron fist - not because they really want to, but because the clash between the ultimately lawless nature of reality and their own internal drives as beings of inflexible order and uncompromising law demands it.
Mortals unfortunate enough to fall under marut control, which has happened in places that fell in debt to them, quickly learn that they are the most brutal and repressive of taskmasters. Completely lacking every single vulnerability, need or urge that mere mortals have, such as the requirements of food and sleep or the limits of memory, maruts demand blind adherence to every rule, no matter how small. Those who must live under their fist learn quickly that there is no pity, no compromise, no mercy; Obedience or Death, that is the only choice.
Ironically, when maruts returned to 5th edition in "Mordenkainen's Tome of Foes", they actually borrowed a fair bit from their 4e lore. In this edition, inevitables have returned, to some extent, but they aren't detailed beyond the barest details. Maruts, and other inevitables, are once again celestial golems, created by Primus, the god-mind of the Modron race as enforcers. In this edition, Primus has created a building in Sigil called the Hall of Concordance, for the purpose of promoting law by forging unbreakable contracts. Maruts (and other inevitables) exist to enforce these contracts, and as such a marut-enforced contract is regarded as completely binding.
The Hall contains a singular construct-engine, the Kolyarut, which takes in gold and the terms of a contract. If the contract meets the Kolyarut's standards - which is to say it contains no vague, contradictory, or unenforceable terms - it transmutes the gold into a single disk, on which it scribes the terms of the contract. This golden disk is then inserted into a hollow in the chest of a marut (or, presumably, other inevitable), where it activates that particular unit, which henceforth exists solely to enforce the letter of that contract. If either party breaks the terms, the marut will seek to punish them, using whatever level of violence is needed to subdue them so they can be dragged back to the Hall of Concordance to face punishment. Or just kill them, if that's what the terms dictate.
For the first time, maruts have undergone a genuine change of appearance. Heightening their resemblance to modrons, 5e maruts appear as armor-plated giants of clockwork innards, with no head but a prominent hunch where the head should be, into which is set a single, ever-staring, organic-looking eye. Below this is the indentation into which the marut's golden disk is inset in order to activate it.
|The inhabitants of the Planes of Planescape|
|Upper Planes:||Aasimon - Angels - Animal Lords - Archons - Asuras - Eladrin - Guardinals - Lillend|
|Middle Planes:||Formians - Githzerai - Inevitables - Marut - Modrons - Rilmani - Slaadi|
|Lower Planes:|| Alu-Fiends - Baatezu - Bladelings - Cambions - Demodands - Erinyes - Hags |
Hordlings - Imps - Kytons - Marilith - Succubi - Tanar'ri - Yugoloth
|Transitive Planes:||Astral Dreadnought - Githyanki|
|Inner Planes:||Azers - Elementals - Genies - Grues - Mephit - Salamanders - Sylphs|
|Sigil:||Dabus - Cranium Rats|
|High-ups:||Archangels - Archdevils - Archfey - Archomentals - Demon Princes|