The Curse of Graush was term for the physical and mental changes that a being possessed by the spirit of Dathka Graush experienced. Sometime before the year 7000 BBY, Sith King Dathka Graush had his heart replaced by a dark side Force-sensitive crystal using Sith magic and the most advanced technology. After Graush's assassination, the crystal was buried along side his remains in his tomb in Korriban's Valley of Golg.
Exactly six months before the Battle of Yavin, the captain of a pirate freighter, Naz Felyood, disturbed Graush's crystal heart and was briefly possessed by Graush's spirit. The experience caused his body to stop aging. Felyood also healed ten times faster than an average being and become highly resistant to physical damage from energy weapons, vibroweapons, and most other melee weapons. Furthermore, the residual dark side energy allowed Felyood to be able to speak, read, and write ancient Sith.
Appearances
Sources
"Korriban: Planet of Lost Souls" (original article link) on Wizards.com (content now obsolete; backup link)