There's an old thread on Dragon's foot about B/X Vs Rules Cyclopedia D&D
but the truth is that the Rules Cyclopedia isn't really complete in one package. That is all of the demons, underworld, plane jumping stuff that I love so much isn't in there.
So what the heck I'm I going to do? Well its not as much of an issue as one my think with the OSR to the rescue!
Ezra Lin fallen Amazon daughter of Thasaidon lord of the seven Hells. Fifth level anti paladin & leader of the Legion of the Wretched. She is working with the raiders outside the lands of Lomar in the service of her dark lich king masters. She carries a sword containing the soul of her former husband & a piece of his demon preserved flesh as part of the sword's guard. The magical sword's power at this time is unknown. She has the scalps of twelve Hyperborean commands adorning her armor.
More on Ezra Lin as she develops but that's a brief thumb nail sketch of this NPC presence at this time. There's lots more to come.