The battle near Grumentum (90 BC) was an setback suffered by the Romans in Lucania early in the Social War.
At the start of the war a Roman army under Licinus Crassus (father of Crassus the triumvir) campaigned in Lucania, in the south of Italy. He was attacked somewhere near Grumentum by an Lucanian army under Marcus Lamponius, and defeated with the loss of 800 men. In the aftermath of the battle, Crassus was forced to take refuge in Grumentum, a hilltop town in the centre of Lucania. A fragment of Diodorus probably includes a reference to this battle, in which Lamponius is reported to have charged Crassus because he believed that the 'people ought not to fight for the leaders, but the leaders should fight for the people'.
Lamponius was something of a survivor. He appeared once again, still fighting against the Romans, in 82 BC during Sulla's Second Civil War. The Samnites and Lucanians had sided with the anti-Sullan forces in 87 BC, and had enjoyed several years of peace, but when Sulla returned to Italy in 83 BC this ended. They sided with Sulla's opponents, and Lamponius reappears as one of the commanders of an Italian army that attempted to raise Sulla's siege of Praeneste. He took part in the battle of the Colline Gate, the last-ditch attack on the city of Rome, and was one of the few senior Italian and Marian leaders to survive that battle.
Grumentum probably fell to the Lucanians soon after the battle. Seneca (in On Benefits), recounts a story that probably came from the end of the Roman siege when the place was recaptured (probably during Gabinius's campaign in Lucania). Late in the siege two slaves deserted to the Romans. When the city fell they rushed to their mistress's house and rescued her from the sack by pretending that they were driving her out of the city to punish her for their treatment. Once they were outside they hid their mistress until the soldiers had calmed down ('resumed the manners of Romans') and the sack ended. Their mistress was then free to return to the city, where she freed the two slaves (it also possible that this incident happened during the Italian siege, as by Seneca's time the former Italian allies were long established Roman citizens). Crassus must have escaped, as he was killed during Sulla's First Civil War, after the fall of Rome to Marius and Cinna in 87 BC.