Tiamat is the Dragon-Goddess of Darkness and patron deity of the dark world of Tethil. She is classically viewed as antagonistic to the Sisters of Light (Ilia and Arteline).
As the original Tethilian succubus, all others such as Mercedes derive from Tiamat. In addition to succubi, there are other "Tethilian devils" such as Dark Children who have received her blessing but are not succubi (though there is at least one Dark Child who is also a succubus.)
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