Thanks to his courage, justice, and ability to exploit the beliefs of the native people, he gained wide support and took over the rule of Spain by force in 80 BC. He acted as a Roman provincial governor and attempted to Romanize the local population.
Roman general and statesman (Died: 72 BC) If circumstances had been different, Sertorius would probably have been one of the greatest Romans. He was a brilliant leader, a master negotiator, and a progressive politician, and supported Marius in his conflict with Sulla.
In the 80s, he led the army sent to Spain to continue the war with Sulla's followers. Sertorius defeated the Roman armies sent against him for years. He also established a new republic there with a separate senate and government.
Quintus Sertorius (c. 126 – 73 BC) was a Roman general and statesman who led a large-scale rebellion against the Roman Senate on the Iberian peninsula. He had been a prominent member of the populist faction of Cinna and Marius. During the later years of the civil war of 83–81 BC, he was sent to recover the Iberian Peninsula. When his faction lost the war, Sertorius was proscribed (outlawed) by the dictator Sulla. Supported by a majority of the native Iberian tribes, Sertorius skillfully used irregular warfare to repeatedly defeat various commanders sent by Rome to subdue him. He was never decisively beaten on the battlefield and remained a thorn in the Senate's side until his murder in 73 BC.
The native Spanish people benefited greatly from his gentle but effective rule. In 76, Marcus Perpenna, who claimed to be a member of the Populist Party in Rome but was actually an agent of the Aristocratic Party, nominally joined him.
Perpenna, who was a supporter of Sulla, waited for the right time and murdered Sertorius at a feast. It was a bitter end for a Roman who was capable of upholding law and order in Rome.
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Quintus Sertorius: Rome’s Forgotten Rebel and the Sertorian War
https://www.discoveryuk.com/monarchs-and-rulers/quintus-sertorius-romes-forgotten-rebel-and-the-sertorian-war/