The Unsullied Heart is a really valuable item that you need to create Greater Blood Essence, which is very important to keep your castle running. However, you need to farm a lot of this item first.
V Rising Unsullied Heart Farm and Greater Blood Essence crafting recipe guide
At one point in the game you will need Greater Blood Essence. This allows you to craft a handful of items like the Servant's Coffin as well as higher quality equipment that you need to progress.
However, to create a Greater Blood Essence, you must first obtain four Unsullied Hearts.
The problem is that this item is a rare find. Therefore, you need to look for an efficient way to grow a lot of it. Luckily, I have the right guide just for you.