The Idirans are a major galactic race, most known for their war against the Culture. By the time of Consider Phlebas, they are an aggressive but calculating warrior species which considers it their holy duty to bring order to the universe and its lesser races.
Full-grown Idirans stand about three meters tall on a tripod of legs and have two arms. There is some hint of fully trilateral symmetry in their ancestry, as a third, vestigial, arm has evolved into a chest-flap which the Idirans use to create loud, booming warning signals. They have a saddle-shaped head with two eyes at each end of the saddle.
Idirans are biologically immortal (or, more correctly, ageless) and are very resilient to physical damage as they are protected by a natural keratinous body-armour and can withstand catastrophic damage and even remain conscious, though they do not naturally regenerate. They are dual hermaphrodites, each half of a couple impregnating the other. After one or two pregnancies Idirans lose their fertility and develop into the warrior stage, reaching greater size and weight, the armour hardening fully. Idiran warriors are capable of taking enormous amounts of damage and can survive massive trauma that would kill a human being instantly-for example, losing a large fraction of their head.
The biological immortality was a result of their evolution as the 'top monster on a planet full of monsters', where strong natural selection pressure and a strong background radiation (causing mutations) prevented the biological immortality from stifling the evolution of the species.
Once the Idirans had tamed their environment they lived in peace and solitude for forty-five thousand years until they were almost made extinct by alien invaders. In response (and in reflection of their physical change from breeder to warrior), they turned into a warrior race and attempted to conquer and convert all other races in the galaxy to bring about the order their God desires (see 'Society' below). This successful and brutal expansion eventually resulted in the Idiran-Culture War. Their religious need to defend and hold once-conquered territory at all costs is described as having been part of their downfall, competing with the Culture's spaceborne flexibility.
Idir, the homeworld of the species, was never conquered during the war, though the Culture succeeded in removing the artificial restraints holding back the development of the planet-wide information network, which then upgraded itself to sentience, 'becoming a Culture Mind in all but name'. This and the loss of the war itself precipitated major changes in Idiran society.
The Idirans are deeply religious and believe in a single, rational God who wants a better existence for his creation. Everything in life has its place and it is desirable to bring about order by putting things into their right places. This belief developed while they were struggling for survival in the harsh and chaotic conditions of their home world. Idirans also believe that they are the only beings with immortal souls - as other species do not even possess biological immortality, they see no reason to assume they would possess the spiritual kind. In this way, they treat all other sentient races as similar to very intelligent pets.
By the time of the later novels, the Idirans have become 'Culturized' to some degree, with some having joined Culture ship crews.
The Medjel are a 'companion' (or slave) species to the Idirans.