The first rules expansion for the Solo Adventurer's Guide, Legendary Characters provides guidelines for playing Solo D&D with only one character (instead of a party) and without the need for complex encounter balance calculations.
A recurring question among Solo D&D players is whether it's possible to play an adventure or even a campaign with a single PC (Player Character), and the answer is yes, it's possible. However, as explained in the Solo Adventurer's Guide, the encounter tables and chalenge rating of D&D creatures are usually designed for a party of four PCs, which makes it somewhat difficult to create balanced encounters, and more often than not, the brave and lonely PC ends up doomed to a tragic and untimely end.
The purpose of this supplement, therefore, is to delve a little deeper into this issue and present another, more sophisticated and interesting solution that makes it possible to play planned adventures for four PCs with a single PC of the same level, without having to recalculate the encounters, reduce the Challenge Rate (CR) of the monsters or the Difficulty Class (DC) of the tests to be performed.
- from the publisher's blurb