|Magic Marker Series||
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Solo modules developed for the Dungeons & Dragons Basic Set and Expert Set, respectively.
Both these were assigned the "M" module codes seemingly without much forethought. When the D&D Master Set came out a few years later, TSR apparently decided to go ahead and re-use the M codes for its Master modules. To dispel this confusion, we'll refer to these modules using the MSOLO codes, even though they were never actually assigned or printed with them.
Both modules come with "magic pens" (either with a blue or yellow cap), which reveal "hidden" text blocks depending on your choice of actions. As such, they are pretty much single-use only. TSR attempted to solve this deficiency with their later solo module, MV1 Midnight on Dagger Alley, which uses a red-plastic "magic viewer" instead. Interestingly, most (all?) foreign editions of the MSOLO modules also use this "magic viewer".
The missing entries of MSOLO1 Blizzard Pass can be found here. Thanks to Benedict Lnu for the transcription!
The missing entries of MSOLO2 Maze of the Riddling Minotaur can be found here. Thanks to Vince Lethal for the transcription!
Contributor Daniel Thingvold notes that the entries to both modules can be revealed by a standard black (ultraviolet) light.
Neither module has any distinguishable
Because of the included "magic pens" (attached on the side of the front cover with a piece of Velcro), and the one-use text blocks, these modules are only rarely found in true-mint or shrinkwrapped condition.