The Necromancy Ring is a platform of the Battle Tower. It is staffed by either Jonelath or Igor depending on who you chose for the quest Caught Undead.
By speaking to your necromancer, you can create your Creature out of the creature parts that you have found. Your creature uses half of your maximum mana to summon.
Base Platform
The initial platform has your alchemist as well as a locked chest & a Dragon Skill Book.
First Upgrade
After completing the first quest, Sassan will allow you to upgrade your platform which will boost your Creature's melee or magic resistance depending on who you choose to serve you.
Second Upgrade
After completing the second quest, Sassan will allow you to upgrade your platform to further boost your Creature's resistances.