Magnaguards were NPCs, presumably cyborgs, working for Valorum/General Grievous. They were distinguished by the magnastaffs they used as weapons.
Revelation of the Sith[]
Two Magnaguards were present with Grievous when he and the Separatists boarded Count Dooku's dreadnought when it was about to leave Coruscant after Dooku had kidnapped Chancellor Palpatine. After Obi-Wan, Anakin, and R2-D2 had freed Palpatine (and after Anakin had murdered Dooku) they were captured by Grievous' Combat Cyborgs and brought before Grievous and his Magnaguards.
The cyborgs took the prisoners to their leader, General Grievous. When Obi-Wan referred to Grievous' "droids", Grievous corrected him. Inside each of these casings was an organic brain, that the "droids" were actually Cyborgs. "Cyborgs. Under. My. COMMAND. Upon hearing this familiar phrasing, Obi-Wan realized that Grievous was the former Chancellor Valorum. Grievous claimed that old name no longer had any meaning to him. Valorum/General Grievous claimed he had ways to improve R2's robotic body. At first, R2 liked the idea until Grievous explained that his idea of improvement was a brain transplant. Grievous intended to give R2's robotic body Obi-Wan's cerebral cortex and Anakin's amygdala. R2 panicked, and tried to zap at Grievous with his ineffective shock probes. This provided Obi-Wan and Anakin with the distraction they needed to use Force Pull to retrieve their laser swords and destroy all of Grievous' Combat Cyborgs and Magnaguards present.
Obi-Wan later confronted Grievous on the planet Utapau. Grievous was unsurprised to see Obi-Wan, and began monologuing. Two Magnaguards and some Combat Cyborgs were also present. The two Magnaguards interrupted Grievous and declared their willingness to take on Obi-Wan. But Grievous was so enraged that they interrupted his monologuing, that he dropped a huge weight on them, crushing them to death.