Just a thought... although I haven't played it, I watched a video of the forgotten one boss from Lament of Innocence and I noticed many similarities... red under the ribs you have to attack, arms that attack you, head that breathes fire. I've also been thinking about how the final boss of Getsu Fuma Den is referred to as The Dragon. And how Dracula means the son of the dragon... not to mention Gabriel referring to himself as the dragon after acquiring the forgotten one's powers.
There are so many similarities between Getsu Fuma Den and Castlevania and since it was included in Harmony of Despair I think it might actually be considered canon. I don't think it was a coincidence that The Forgotten One was made an optional secret boss in Lament of Innocence and a DLC boss in Lords of Shadow... and Getsu Fuma Den was DLC in Harmony of Despair....... interestingggg.
Anyone else have any thoughts on this? In the enemy data for Harmony of Despair, Ryukotsuki is described as "A fiend who hails from the deepest pits of hell."