The fifth and final mansion is Laruba Mansion, in the middle of a poison swamp.
Go to the Center of the Mansion
From the entrance, go to the right until you see some stairs. Go up those stairs, then go to the left and go up the stairs there. Jump up to the ledge on the right, but ignore the stairs and keep going right. You will eventually find more stairs, but ignore those as well. Go to the right to reach the next area, where you will see some more stairs.
Get an Oak Stake
If you need an Oak Stake, go up those stairs and follow the path until you find the Gypsy who is selling the Oak Stake. From there, go to the left again, and go down the stairs (or just walk off the ledge and you will land at the base of those stairs).
Get Some Free Laurels
If you don't have the maximum number of Laurels in your inventory (4 if you don't have the Silk Bag, 8 if you do), then go downward from here until you see a stone column in the background. Go downward along that column until you can see some moving platforms down below. Jump off to the left and you will find a Gypsy who begs you to take some Laurels. He will give you the maximum number that you can carry.
Go to the Boss
Go to the right from the Gypsy, jumping across the moving platforms while avoiding the spikes that shoot upward, and keep going toward the bottom-right of the area. Go right from there and you reach a boss. This Weeping Mask will drip fireballs to the floor, which will shatter into fireballs that shoot in several directions. Just equip Dracula's Rib and wait for the Mask to start spinning around. While it is spinning, attack it. Repeat this process until the Mask dies, leaving behind the Magic Cross. Take the cross, then go to the right to reach the chamber with the piece of Dracula.
Get the Piece of Dracula
Jump up to the platform to the right and throw the Oak Stake at it, then jump onto the bag to get Dracula's Ring. This item doesn't do anything when equipped, but you need it in order to get into Dracula's castle.
Leave the Mansion
Go to the left. The Mask is back, but just ignore it and go to the left. Jump up to the floating platforms and keep going up until you can't go any farther. Then go to the left, where some Tarantulas will attack. Keep going to the left until you can go up. Keep going up until you see some stairs. Ignore the stairs and go to the left. You will find more stairs. This time, go down the stairs. Then you can simply go to the left until you are out of the mansion.