After getting the Blue Crystal and using it to go under Yuba Lake, you arrive at Rover Mansion.
Find the Gypsy
As with Berkeley Mansion, you will need an Oak Stake in order to get the part of Dracula in this mansion, so you first need to find the Gypsy.
Go right from the entrance, and go up the stairs. Then go to the right and hop onto the first platform. Then walk off the right side of the platform to land on some blocks below. There appears to be a wall to the right, but it's actually fake, so just jump to the right to land on the lower blocks.
Continue going to the right, jumping over the water. Go past the stairs, then jump onto the platform over the water. Be sure to jump over the moving platform so you don't get pushed off into the water. Jump to the area to the right, and jump over the spikes, to find the Gypsy. Pay him 50 hearts to get an Oak Stake.
Get the Piece of Dracula
From the Gypsy, go to the left, jumping over the spikes. Jump up to the platform, and jump over the moving blocks again. Go to the left and jump down to the lower area. Go to the left, then go up the stairs. Keep going up until there are no more stairs. Then go to the right, where you will find more stairs. Go up those, then go to the left. Carefully jump onto the block on the left (if you fall down below, just go up the stairs and try again), then go upward from block to block.
When you reach the top, go to the right and go down the stairs. Then go to the left and go down the hole until you are at the bottom. From there, go to the right to find the chamber with the Dracula piece. Go up to the orb and equip the Oak Stake, then throw it at the orb. Jump onto the bag, and you will now "prossess" Dracula's heart. The heart doesn't do anything when equipped, except in one specific situation that we will see later.
Leave the Mansion
It's time to leave the mansion, so go to the left and jump up the series of blocks until you are at the top. Go to the right and go up the stairs, then go to the left and fall down the hole until you are at the base of some stairs. Go to the left and jump up the platforms, then go down the stairs at the end. Walk off the ledge to the right, then go down the stairs there. You should go to the right from here and buy another Oak Stake. Then go to the left until you see a wall to your left. This is the fake wall that you encountered earlier. You are going to have to make a difficult jump over the water onto the raised blocks to the left. Once you make it, jump down to the left and keep going left to leave the mansion.