Undercommon

Yaslir Vicloth claims that her mother Lady Arasta created Undercommon by combing Abyssal and Elvish into one language.
This could explain why the Drow and Drider uses Undercommon as their main language and why the Undercommon alphabet looks like someone twisted the Elvish alphabet and been spoken to in Undercommon sounds like threat been song.
Note: I sadly haven't had must opportunity to travel the world and can therefore only believe father word about Undercommon originating from Nix.