The Nightblade was a melee weapon crafted by the natives of Utapau. It was sometimes referred to as an Utapaun Nightblade.
The N-K Necrosis droid, built using the lifeless, cyborg body of General Grievous, carried an Utapaun Nightblade as part of its arsenal. When the Necrosis droid was destroyed in the Myyydril Caverns of Kashyyyk by a group of spacers, its possessions were looted, including the Nightblade.
Sources
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"The Story of General Grievous: Lord of War" on Hyperspace (article) (content removed from StarWars.com and unavailable)
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