[Source] Through the use of Sith magic and alchemy, the Sith King Dathka Graush replaced his own heart with a Force-sensitive crystal later called the Heart of Graush. The Heart of Graush was made of the same material as the crystals within Sith crypts that housed the spirits of vanquished Jedi Masters. Within the heart were the dark side spirits of thousands of long-dead Sith. Graush used the Heart to make himself almost invincible.