Little is known about Greywyn's life or past, save that he was a member of the Dark Brotherhood. Around 12 years before the events of The Elder Scrolls IV: Oblivion, he believed that Sithis had instructed him to convert or purge all non-vampire members from the Brotherhood, and started gathering like-minded supporters. He called his faction the Crimson Scars. Within days, however, he was betrayed by a Scar named Silarian. The Brotherhood attacked him, and he barely escaped, unaware of the fate of his followers.
Greywyn fled to Deepscorn Hollow, where he waited for his allies and began preparing the den to act as a base for the reformed Scars, with the help of Scar and former merchant Rowley Eardwulf. Around two months later, he received another vision from Sithis, this one seemingly condemning his actions and demanding that he cure himself of vampirism. He set about finding a cure, and managed to acquire the Purgeblood Salts. It is unknown whether he succeeded in curing himself.
Greywyn does not personally appear in any games. He is mentioned in the Vile Lair add-on for Oblivion, where the Hero can discover his journal. Reading it reveals that he is mostly, if not entirely, responsible for the contents of Deepscorn Hollow, such as the Shrine of Sithis and the Purgeblood Salts.