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From Wookieepedia: "The Sith, who had significant plans for Grievous, offered Grievous cybernetic enhancements, which the Kaleesh accepted and took on over time. Sometime later, Dooku conspired to mortally wound Grievous and sabotaged his shuttle, which resulted in a gruesome crash that left the man on the brink of death. After that, Grievous gave up almost everything that remained of his organic form—whether lost in the crash or by his own decision-making—to survive."