Death's Door

This can be unlocked as soon as you get the fire spell if you so choose by heading back to the entrance of the Estate of the Urn Witch and clearing out cobwebs with your fire spell that previously blocked your path. You will have to defeat a mini-boss through the upgrade door, which will give you the upgraded fire spell that adds a damage-over-time effect to the spell - burn baby! Reference this video at 2:27 to find the location: