From the website:
Products of generations of Everblight’s manipulations, succubi are fonts of blighted arcane power. To create each one, a specially selected Nyss is brought to the spawning pools and ritually bathed in the blood of a warlock. It emerges as something no longer wholly Nyss, attuned to the warlock whose blood actualized its second birth. This ritual empowers each succubus as an amplifier of its warlock’s own gifts. With their every thought and action these creatures carry out the will of their masters and the dragon itself.