Just something I thought up when considering the preponderance of mortal-level demons in later editions of D&D. Enjoy!
Durga Mother of Demons
by Rodger BurnsDurga, called the Mother of Demons, the Forgotten Goddess and the Unseen Elder, is an ancient Immortal of Entropy. Though not much known among the civilised races of Mystara, she is greatly respected and feared among the ranks of her Immortal peers for her patience, her ruthlessness, and her insight into the best ways to corrupt or destroy mortal cultures and civilisations. Even among the Entropic immortals, Durga is considered insane - she offers not even the facade of friendship to her tools, and will sacrifice her purported allies without a second thought if doing so advances her goals.
Durga is also unusual among Immortals for her apparent unconcern for the Prime Plane and the normal matters of Immortal society - she maintains no congregations among mortal races and doesn't grant spells to clerics, avoids interaction with other Immortals, and doesn't execute Plots. She's instead dispersed her attention and consciousness across the depths of the Outer Planes, seeking out places that even her fellow Immortals rarely dare to enter, and it's rumoured that her mad daring has been rewarded with prizes of great power that she quietly manipulates to spread death and chaos.
Durga's ultimate intentions are unknown, but it's thought that they begin with the violent destruction of civilisation and established order throughout the multiverse and get worse from there. Any such plans would no doubt take centuries or millennia to achieve, but Durga has time, and quietly works to turn her enemies against one another and weaken them for her eventual triumph. She is known to have created races of lesser demons - mortal beings cast in the shape of Entropy's favoured servants, but much less powerful and able to reproduce without the use of Immortal Power - and filled entire planes with their festering hatred, out on the edge of the multiverse. Other instances of unseen corruption throughout the multiverse may possibly originate with Durga, particularly if they harm other Entropic Immortals or seem entirely without purpose.
Durga's avatar is a man-sized blot of utter shadow, normally formless but able to reshape itself at need into a perfect ebony visage of any type of demon. Her avatar is functionally invisible in any location not lit by sunlight or continual light.