Worlds Without Number

One of the best solo-play Fantasy games is the incredible Worlds Without Number game by Kevin Crawford. This book has so many tables to generate worlds, campaigns, and adventures that, as a solo player, you will never run out of things to do.

And it is just one book. No library needed. No computer programs required. No shelf of supporting books needed. Just this one book, and the world is yours.

The rules are simplified BX, and there is a one-page summary. All combat is d20 versus AC, and damage is rolled as you are used to. All skills use a 2d6 system, which is different and cool, and I am not so hung up on the idea that everything needs to be on the d20, since the original BX game had 1d6 skills.

Spellcasting classes are different, but they have more power than their BX counterparts, especially with arts as powers that can recharge their effort pool between encounters. You can be a "shooty" wizard here and fire off elemental bolts a few times during a fight, and have them back for the next. Magic is powerful and cool here.

Different here is fantastic, and you can "port in" spells from BX if you want to, along with magic items, monsters, treasure, and anything else you could imagine. You can use BX monsters and adventures right from their books.

Oh, and there are heroic rules too, if you want to play a larger-than-life solo hero. Or if all your players want to, in case you play with a group.

There is a world here, and plenty of world-specific classes and support, should you choose to play in this lost civilization setting. This is a "billion years in the future" fantasy world, where all is forgotten, the land changes, and the world is an unknown and mysterious place, rooted in traditional swords & sorcery tropes and races. You can even have "elves and dwarves" if you wish, and more exotic and strange kin walking around. You can say your world is more traditional fantasy, and forget the Lost World tropes. It is all up to you.

The random charts and tables make this game special. You can create your random heroic character, pick up a sword, and have random adventures and explore random lands until your character retires as a king or falls as a hero. You can make random factions, magic items, NPCs, adventures, and anything else your world needs.

Forget D&D for solo play, and even BX - this game has it all, and can port most anything into it to play with. This is one of the highest recommendations for solo play, and it is a fantastic core system in its own right.