You are the new Dark Lord, also known as the Prince of Darkness or Demon King.
As the Dark Lord, your will extends across all Devils/Demons, allowing you to bend the demonic spirits to your command.
This has enhanced your power of Dominance, making it so you no longer have to just defeat individuals in combat. Instead, you can now have individuals swear themselves to you, binding their souls to you and Castlevania, tying their very existence to your power. They become as much a part of you as Castlevania has become. This allows you to do as you please with them, such as giving them access to any abilities you or those bonded to Castlevania have, changing their form, reviving them if they are killed, or even erasing their very soul from existence. It's up to you.
Your bond with Castlevania has become complete, making the Castle now an extension of you and your power. The Castle can freely change its form to suit your desires and control its interior dimensions as you wish. It also makes you aware of all that happens and is happening on its grounds, acting as a focus for your powers. Moreover, it can revive or recreate those in your power or service through the Castle if they die.
If Upgrade you also gain the Knowledge & experience of all the previous Dark Lord's