From publisher blurb:
A Horrifying Race of Creatures for 5th Edition Dungeons & Dragons
Modrons are native to an eternal, unchanging, perfect clockwork dimension, where they unceasingly care for the mechanisms upon which the universe is balanced. But these are not the only modrons. There are also modrons in the nether plane, the place where law is tinged with – but not yet totally corrupted by – evil. The nether plane is a dimension at constant war, where armies march in ceaseless battle for control of lands or resources, and in this dimension no army is more dangerous than the modrons who have recently made this place their home. The modrons of here are known as Tainted Modrons, and they are very different from the others of their kind, in both appearance and attitude.
Modrons of the Nether Plane includes the following:
A history of these evil clockwork creatures
15 new monsters for your Dungeons & Dragons 5th Edition game, ranging from mere nuisances to godlike foes
Nine short adventure ideas
A full adventure for a party of 6th-8th level