Living Constructs are relatively rare on the planes, being a special variety of an already uncommon type of creature. In short, they are Constructs that have sentience and free will, and that combine aspects of both constructs and living creatures.
Since, like constructs, they are created, this usually means their creation entails powerful and complex enchantments, usually on the magical scale of deities or artifacts.
Probably the most common living construct on the planes, and also the oldest, are the modrons. The exact origin, both method and date, are unknown to scholars at this time and are likely to remain such, due to the general difficulty of interacting with modrons in a sensible manner.
The other main living construct "species" is the warforged, though they are much more commonly encountered on their Prime worlds of origin, the most well known of which being Eberron. Warforged of other primes are also sometimes found on the planes, but the impetus to create them has yet to be reached due to various environmental factors.
Other than these two peoples, living constructs can occur on an individual basis, none really widespread enough to count as a independent species. To the consternation of scholars, this is usually a coincidental occurance &emdash golems that develop sentience over a millenia-long existance, freakish magical transmutations, or deific fancy &emdash though some great crafters are reported to have created living constructs of their own, usually as a lifetime endeavour.
The most notable features of living constructs are those arising from their construct heritage. They have no need to sleep, eat, breathe, and are not affected by mortal frails such as poison, tiring, or old age, though they suffer from wear and tear over the ages. This remarkable durability comes offset with a price: unlike living creatures, they do not heal naturally, so while they are more resistant to harm, injuries are much more long-lasting and harder to fix with curative magic. Indeed, many encounters with living constructs are initiated when a planeswalker finds and repairs a battered construct from critical or almost-destroyed condition.
The other main feature of such peoples is that they do not reproduce naturally. Being constructs, they must be created, and so far there have arisen many different means of creation. The primary means is through artifacts, such as the Creation Forges of Eberron which allow for mass production of warforged to fight in their war. Individual living constructs can have wildly varying origins, from deific curses or boons to wild magic effects to mad alchemy gone horribly right. The creation of the modrons is a mystery; some claim Primus creates them, others say that the Energy Pool is an incredible powerful artifact that keeps their numbers constant.
- Eberron Campaign Setting, pp.20-24
- Races of Eberron, pp.7-24
- Dragon #354 - Return of the Modrons, pp.35-49