Infernal Exalted, sometimes called akuma, are Exalted whose Spark of Exaltation and soul are devoured and regurgitated by a Demon or Yozi, resulting in a creature with the power of an Exalted and now devoted to the creatures the Exalted were created to slay.
Almost nothing substantive is known about the Infernal Exalted (there is one example, Dukantha, given in Blood And Salt). However, it is known that they become an attendant soul to the demon that devoured them, giving them power equivalent to a soul of the corresponding circle. So an Infernal created by a Yozi would be a Third Circle Demon, one created by a fetich would be Second Circle.
There are various ways to create an Infernal Exalted, although they are so incredibly rare that any method can be considered unique. The as yet unwritten Broken-Winged Crane is a common approach, other methods involve tainting existing Sparks, as the Yozis are likely doing with Sparks acquired from the Malfeans.