Special Children Of Aran

Special Children refers to children to whom Aran fed his blood when they were babies, having snuck into their homes after making a deal with their parents.
 * "My plans for you, Phoebe. You... and all the children like you."
 * —Aran in Death's Trap

The result was that the blood caused the children to develop supernatural abilities in them so they could serve as generals in Satan's army against humanity and God. These abilities could be strengthened by the consumption of demon blood.

Around book 4, Aran wanted to test these children to find who would be the strongest, so he put them in an abandoned town called Cold Titanus and told them only one could survive. This eventually triggered a fight among the Children. Aran led everyone to believe that he wanted a leader for his demon army.

However, as it was later revealed, the endgame was bigger than just leading a demon army. Aran chose the children because they were all potential vessels for Satan. The strongest one would be chosen to contain Satan throughout the Apocalypse. Phoebe Llewoh was the sole survivor and so, despite her greatest efforts, was taken over by Satan both as a result of being a Special Child and also because of his predetermined destiny as Satan's true vessel.