Bio-mechanical supercomputers built by the enigmatic Ancients of Rain World. Their "bodies" consist of a massive supercomputer and an android-like "puppet" which acts as an interface. Any damage done to the computer appears to be felt and expressed by the puppet.

The iterators were built during an era known as the Void Fluid Revolution, brought on by the discovery of "void fluid," a substance that, among other things, could allow a creature submerged in it to "ascend," leaving the physical world behind. however, void fluid could not be used to ascend creatures that weren't "enlightened" enough, such as less intelligent and instinct-driven animals. The Ancients called this the "Great Problem," and tasked the iterators with finding a solution.

After the mass ascension of their creators via void fluid, and being unable to ascend themselves, their duties were twofold: to ponder for countless years on an alternate means of ascension, and to spread the wisdom and enlightenment of their people to lesser beings (like the Slugcat) so they, too, could fling themselves into the void fluid.