Succubi, or sayaad, are agile fighters with command of the mystic arts—particularly the ability to magically charm their targets. While there are many sayaad that loyally serve the Burning Legion, there are also those that serve only themselves or can be impressed by a warlock master.
As the succubi are demons, they had been formed as a result of the Light and Void energies that had bled together at the borders of the Twisting Nether. Kil'jaeden brought in the devious succubi to infiltrate worlds for conquest and gather intelligence about their civilizations.
Professions of the Cosmos
Warlocks were former mages (or shaman) that delved too deeply into the roots of demonic power. As a warlock's knowledge grows, seductive succubi join the dark sorcerer's ranks to wreak havoc on anyone who stands in their master's way.