I don't think Nyst was ever a "natural" being at all, just a sort of intelligent Tower construct of the same type that Xerces "army" in his confrontation with Hester was composed of.
I tend to agree with Decanter's "crazed elf" assessment of the AoA.
I posted this elsewhere a while back, but remember that the extant sentient races in TLS at the game start aren't the only possibilities for Dio/the Lustlord. Tertia's intro speech implies the cycle of Incubus Kings and Goddesses has gone on for many, many millennia, back into evolutionary and geological time. The Lustlord was apparently kicking around for at least a thousand years, It's possible he was the last member of race that is otherwise as dead as the dragons.