Fiendlings were mortal creatures who had their usual ancestry mutated by some influence of the lower planes. They were one of three types of planetouched. Fiendlings might have been the result of an ancient blood debt to a devil, a demonic possession, or even just the whims of influential evil gods. Although they could be born to any other ancestry, humans were the most easily corrupted by fiendish influence, making fiendlings most common in human societies.

Fiendlings were created by planar interference and might have been born to any other ancestry, usually bearing no similarities to their parents. They stood between five and six feet tall and had builds similar to humans. Fiendling eyes were a single solid colour with no pupil and could be black, silver, or gold. Regardless of their parentage, fiendlings matured in 15-20 years and lived up to 100 years before the planar influences that created them faded, killing them. This process was sudden and without warning, and a fiendling would show no signs of their old age.

Depending on the nature of the fiend that caused their creation, other aspects of a fiendling's physical appearance would vary. Fiendlings created by devilish influence had red, pink, or light tan skin and may have had horns and thin tails whereas those created by demonic influence had grey, dark grey, or blue-tinted skin and may have had clawed hands and hooves for feet. A combination of these features could exist when even higher-power fiends were involved, especially the gods of the lower planes.

Fiendlings could have been intentionally or unintentionally created by the gods and godkin of the lower planes. For example, a devil might have intentionally made a deal with a mortal creature which caused their offspring to be fiendlings. Or, a cult performing a demonic ritual might have caused the birth of a fiendling in a nearby village. Even though they were not inherently evil, fiendlings were frequently ostracized by their communities and sometimes even their own families. Early in their lives, many fiendlings heard the whispered words of evil entities from the lower planes trying to corrupt them and could learn their languages that way.

As gods and godkin learnt the power of mortal worship, they began exerting their influence over Aedelor in various ways. The boundary between the outer planes and Aedelor was thin enough to allow them to perceive the world, but still too strong for them to traverse the planes. As the mortals in Aedelor became more aware of the other planes and developed mastery over magic, they too became interested in the outer planes. A crafty devil from the lower planes discovered a way to communicate across the planes and made deals with various people who were too easily corrupted by the promise of power.

The evil and destructive power of the lower planes began to bleed through into Aedelor and tainted the lineage of the people making these deals. Though the boundary between planes was still too strong to traverse, devils planted the seeds for what would become lower planetouched generations later, as the price of the pacts began to come due. Powerful creatures from the lower planes continued to exert their will over Aedelor in places where the boundary was weakest, or by making deals.