Ultima 5 is apparently missing documentation. In the commercial version, Ultima 5 contained a log from Lord British's lost expedition. The log described how to reach the underworld through Spiritwood as well as the path the party followed once they reached the underworld. Either that or I just have been unable to find it in the Ultima 5 downloads on GOG.com
Replacementdocs.com has the journey log for Ultima 5.