A modronoid is an artificial horror created by a deranged arcane spellcaster grafting parts from a base modron to a humanoid. This procedure is enough to drive both the “donor” modron and “host” humanoid completely insane, driven to kill everyone they see and, in their brief moments of lucidity, try to destroy themselves. Implanting part of the donor modron's brain into the host can prevent this “modron madness” to create a sane modronoid that will slavishly follow orders, although this complex procedure can still drive the modronoid crazy and does nothing to improve its intelligence.
The life force of a modronoid is linked to the life force of the modron its parts came from, so if one of them dies the other immediately perishes. The mutilated remains of the modron donors are restrained and kept alive in some secure place so they can't die and destroy their linked modronoid. The modrons too badly damaged to survive by themselves are kept alive in vats of a strange sludge. This life support ichor is probably a alchemical fluid of some kind, although rumors claim it's made from the processed corpses of modrons whose modronoid fusion surgeries failed. That should be impossible though, as such modrons would have their remains vanish back to Mechanus when they die.
A typical modronoid stands 5 feet tall and weighs about 300 pounds.
In Planescape
Modronoids were invented by a barmy wizard named Valran Stonefist, a member of the Society of Sensation faction. Valran worked with the Tacharim, an outcast group of murderous and chaotic thugs, with the ultimate aim of creating an army of modronoids. The base modrons he used in his experiments were abducted from a highly abnormal Great Modron March. Every 289 years, a great host of modrons march around the Great Wheel on a tour of the Outer Planes, but this Great March did not match the 289-year cycle, which was truly strange behavior for the clockwork-like modrons.
Valran Stonefist ended up having his brain implanted into the body of a Decaton, becoming the first
hierarch modronoid. Adventurers reportedly slew the decaton modronoid Valran at The Flower Infernal in Gehenna, the Tacharim's headquarters and main base.
I'll post an empty draft for the Hierarch Modronoid Template and update the link above.
EDIT: Upon reflection, I see no reason why the "brain swap" modronoid template couldn't use base modrons as well as hierarch modrons. Shouldn't the procedure be able to put a humanoid brain into, say, a pentadrone as well as a decaton?