Modron

Modrons are a race of Lawful Neutral outsiders introduced into Dungeons & Dragons in 1st edition (MM2, pg 86), native to the planes of Nirvana. In AD&D 2nd edition and the Planescape setting, they were the primary inhabitants of the extreme lawful-neutral plane of Mechanicus. In D&D 3rd edition's Manual of the Planes, they share Mechanus with the Formians (giant ant-people) when their lawful-good plane went too lawful and was annexed by Mechanus.
Modrons are a race of biomechanical clockwork robots resembling geometric shapes, and with a mental process so regimented and programming code-based they are paragons of of Lawful Stupid. They are goofy, if not outright ridiculous, but also kind of awesome and are perhaps one of the most iconic races invented for Planescape.
Modrons are a hiveminded race, with each member of the race being nothing more than a literal cog in a plane-spanning machine. Divided into almost sixteen rigid castes, physical form and intelligence escalate change from caste to caste. Promotion to a higher caste will change the modron's physical shape and mental faculties, but not personality (if modrons can be said to have any individual identity). The lowest tiers of the caste tree, the "base modrons", are literally not sentient for the most part; monodrones can literally only process a single order at a time. For example: a monodrone could "dig" but could not "dig then pick up gold. Monodrone are so simple that a compound order like this is too complex to remember. Duodrones can process two orders at a time, Tridrones can process three (which makes them just barely sentient), and then you finally get Quadrones, who are the bottom of the sentience barrel for the species, though still dumber then anything that isn't a base modron. At the cusp of base modronss are Pentadrone superiors, the first caste that have enough mind to give commands. After pentadrones, you have the "hierarch modrons", who get increasingly smarter; decatons, nonatons, septons, hextons, quintons, quartions, tertians, secondi, and finally Primus, the god-like embodiment of the modron hive-mind.
How does this race get anything done? Hierarchy of orders. In essence, commands are passed down through the chain of castes from Primus down to the lowest modron, getting increasingly simplified until even the monodrones can do their part. Cogs in a machine. It's so hardwired into them that modrons can't even recognize members of their own species that don't belong to the same caste, the caste directly above, or the cast directly below.
The most awesome modron ever is probably Nordom, the "backwards Modron", from Planescape: Torment.
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From Monster Manual II