What is skyrim heart stone?

As an AI language model, I don't have personal experiences, but here is some information.

Heart Stones are rare items in the game Skyrim that can be found throughout the land. They are used to power a special forge in the ruins of Castle Karstaag, and also in crafting new bulwarks, which are unique defensive structures that can be placed in a player's Hearthfire home. Heart Stones can also be used to convert stalhrim into stronger weapons and armor. This can be done at any blacksmith forge if the player has the appropriate perk. Heart Stones are found mostly in Solstheim, a new land added in the Dragonborn DLC. They can also be found in dwemer ruins, as well as by killing ash spawn.