Mechanus, the Plane of Uttermost Law

Mechanus is sometimes also referred to as Axiom.

The Inexorables

The Inexorables—the Knights of Axiom, the plane of uttermost law—arrive at the summons of those devoted to opposing chaos.
Each Inexorable is charged with enforcing one of the Laws that bind the universe together. They don’t bother explaining this, and the nature of these laws and the Inexorables themselves remains a mystery to most mortals, few of whom have ever heard of the Law of Time—or the Law of Death.
Like the Celestial Court, the Inexorables do not have distinct psychologies, being more a manifestation of a deity. So while they can and do speak, it’s not clear what their intention in the Mundane World is. They hate the violation of law, this much is clear, but their notion of what counts as a “law” is alien to mortal minds. How many Inexorables there are, no one knows. Only those who have visited the Five Cities of Ordos, the capital city of Axiom, can say. It is known that there are Inexorables greater and lesser than those described here.
Magic is Chaos. Each Inexorable has their own Law, and there are many Laws. But one force the Inexorables universally hate is magic. Magic is chaos made manifest. Magic is the tool chaos uses to disrupt the world. Almost all the Inexorables are vulnerable to magic. It interferes with the perfect order of their logic and, moreover, it offends them. Its existence is anathema.
Some Inexorables, however, can overcome this revulsion and maintain their purpose in the face of discord. Inexorable. The forces of Law that grant the Inexorables the power to travel the planes, arriving when summoned to repair the damaged fabric of reality, also prevent their duty from being interfered with. When it comes to denying the Inexorables the opportunity to pursue their purpose, even the chaos of magic cannot stop them. The Inexorables are immune to any effects that would slow them or deny them actions or movement.

Space

The Inexorable limitations of Space forbid anyone from moving from point A to point Z without passing through the rest of the alphabet. Certainly nothing as vulgar as teleportation is allowed on Axiom.

Spatial Arbiter. “The fundamental theorem forbids,” is a common refrain whenever Space prevents someone from moving in a manner contrary to the strict laws of motion. A creature under the effect of a haste spell, for instance, gains all the benefits of haste except for the extra movement. They may attack, hide, or use an object, but cannot take a second movement action and dash or disengage.
Space allows a second attack, or a second use of an item, because those are in the purview of Time. And Time may do what is necessary to enforce its Law.

Death

All things must end. The Inexorability of Death is not concerned exclusively with ending life—its purview covers all things. Civilizations, ideas, stories. All things end. Death does not cause this ending, it enforces it. It arrives ready to stop something or someone trying to undo the Law of Death. Therefore, it focuses most on the undead, on those things persisting after their natural end.

Exceptions. It also has the power to prevent a creature from meeting its end before its time. Why it is granted the power to use magic, when magic is chaos and therefore anathema to the Inexorables, is a question Death does not know the answer to. It only knows it can prevent.

“This microstate partition lacks sufficient

entropy. I will repair.”

Change

A close cousin to the Inexorables of Death and Fate, the Law of Change states that what has been done cannot be undone. Change opposes any magical attempt to reverse what is done, such as using magical healing to close a wound. “You cannot rewrite the world.” 

Loopholes. Of course, Change knows who its master is, and being more sophisticated and complex in its order than either Space or Death, Change can make subtle distinctions between friends and foes. The null-entropy field that prevents magical healing near Change only works on enemies.

“Lore is a tool. A tool used in the advancement of

chaos must be stopped. But the pious do work

with lore that benefits order. This I can ignore.”

Fate

The Law of Fate forces events to have predetermined outcomes whether you like it or not. There is no chance when Fate is present. All things proceed as the universe intends, and neither good nor bad luck is possible. There is no Inexorable of Luck. Luck is a force of Chaos. Fate is its enemy. Chance is the enemy of Fate.

Law of Large Numbers. Armed with the Destiny Light, Fate inflicts perfect order on her enemies. Those who fail their saving throw against the beam may still be very effective in combat, as they no longer need worry about rolling poorly... or well. All things thereafter proceed in a perfectly ordered fashion.

Time

Time is something of the hypocrite among the Inexorables, perfectly happy to speed things up for itself while denying that same ability to anyone else. Many of the Inexorables don’t mind if their allies bend the rules so long as their enemies toe the line, and if this seems like commentary on the nature of law and how it tends to be applied, of course it is. Or is it? Maybe it’s the opposite.  

Elvish Enmity. Time enforces the rules of causality and ensures the past remains firmly behind the future in spite of attempts by creatures like the elves to muddy the waters. The Inexorable of Time really doesn’t like elves, by the way. The elves have an annoying habit of ignoring causality altogether.

Nature

Chief amongst the Knights of Axiom is Nature. The Law of Nature states that all actions must proceed according to the fundamental axioms of the universe. Magic is a tool used by those who would bypass those laws and act on the world using their will alone. This cannot be permitted. Nature can reflexively attempt to counter a spell as it’s being cast, pointing at the caster and bellowing, “OBEY!” or “CONFORM!”

Alien Nature. Nature has no particular interest in the local phenomenon humans call “nature.” Plants and streams and such. To the Inexorables, “Nature” is stars and elementary particles. The world Men inhabit is messy but legal as far as the Inexorables are concerned. Until someone starts using magic.