Though bested, Mondain’s reign of terror was not over.  He journeyed to the continent known as the Lands of the Dark Unknown to continue his dark works.  While there, he created dark creatures and unleashed vile magics.  In his pursuit of immortality, he sought to corrupt a powerful gemstone known as the Sun Ruby Gem.  The details from here becometh murkier, but it is known that a Stranger from another world came to Britannia summoned by Lord British to rid all of Sosaria of the threat from Mondain.  This Stranger known to us as the Avatar succeeded in preventing Mondain from gaining immortality by shattering the Sun Ruby Gem and slaying the dark wizard within his castle.