Back to the subject of Heart of Darkness. I am now 100% sure, that we should have it act as a power source, rather than a wish-granting gem. So, whereas Hogyoku grants it's wielder's wishes, the HOD would lend them power to grant their wishes - with a considerable price, as expected from a cursed artifact. Like I said, it could also have it's own powers of manipulation and self-preservation (if it's wielder was to destroy it, it could do something to forbid them from doing so - mental influencing and even death by taking back the power it has lent). The tricky thing about HOD is, that it serves only itself, and all of it's wielders are destined to be bested - in fact, the heart itself could have it's wielder defeated, so that it could find a new owner.
This power could be used to access whatever the wielder wishes. Seach could long for enough power to return to his original body and strike terror in the hearts of his opponents (or something, I totally have no clue about his agendas, but the point is he has an agenda with the Heart of Darkness). Katherine could want the power to resurrect her deceased lover, and attain absolute beauty. Dante would want the power to dominate the lands around him, brute strength and undeniable kingship. Or smthing.
[Only admins are allowed to see this link] Are we introducing Virgil as the Bastard Prince of West? Even if Dante doesn't die, he is either captured or flees to safety his own kingdom cannot currently offer him, and in absence of both Annora (the Princess of West) and Morgoth (highest-ranked general of Dante), Virgil would have the perfect chance of seizing power in the West. There's still Berenguer and the remaining Horsemen, as well as the six of The 7 Deadly Sins, who were never written out, despite their existence as Dante's elite soldiers - Morgoth was the Pride of this group, btw. It looks like the kingdom of West isn't going to give up without a real fight.