I've tried a lot of solo systems, and I keep coming back to Mythic, because every other system just feels like a stripped down version of Mythic. Except the PbtA ones, I guess.
Mythic doesn't really work with D&D, however, since Mythic assumes a cinematic approach to gaming and D&D assumes a simulationist one. In Mythic, game play is divided into a sequence of scenes, where scenes are chosen in respect to how they push the story forward, and do not have to play out in a linear order and all the unnecessary events in between are ignored. This doesn't really fit D&D's simulationist approach, which doesn't divide cleanly into discrete scenes and the "unnecessary" mechanics of the journey are in fact the greater part of the game.
But I think that Mythic can be salvaged for use in old school D&D by hacking off the parts that work and repurposing them; specifically Threads, Lists, Events, and Chaos Factor.
Threads are basically the goals of your character, as an end goal like "killing the demon lord" or something on the side like "finding long-lost brother". Mythic has many tools for managing threads and manipulating them at unexpected times. Creating, following, and resolving threads adds a story-driven direction to the game.
Events are the meat and potatoes of the solo engine. Everything revolves around them and most of the solo game will be spent dealing with the consequences of an event. Use events in place of a random encounter. Perform the random encounter check as normal, and on a success turn to Mythic’s event generation rules and fit what happens. The Mythic GME constantly stresses that Events should be defined by the context of the game, and D&D provides the context, whether overland, in town or in the dungeon.
Lists and List Management are necessary paperwork for tracking and manipulating Threads and NPCs in your game. Only add “important” NPCs to the list, as in D&D you’ll probably be generating a lot. When a thread or event states to “Introduce NPC”, that would be one important to the story and should be built accordingly. The 1e DMG has a table of traits for random NPC generation, which I found to be more straightforward than the Mythic one, but has more limited options.
The Fate check is introduced in the Mythic GME as it’s most important mechanic, but I found it’s usefulness limited in old school D&D, as there are usually more concrete procedures for resolving in-game activities. Nevertheless, there are points where procedures or results are unknown, and it’s there that the Fate check comes into play. I recommend using the Fate check from Variations 2.
Most people who play solo don’t like to use Chaos Factor, but I do. I like the element of randomness and the unexpected that it provides.
I have the Mythic cards, and generally I prefer them for tracking Chaos factor and Lists, but for events and the other checks I like to use Variations 2.
