General Grievous was a Kaleesh warrior born on Rattatak where he fought for his people. He was a befamed warrior that was feared by all. When the republic aided a previously invading planet, Grievous his hatred against the Jedi began.
He allied himself with the banking clan who used him as a weapon against those who didn't pay their interests. Later, he underwent "improvements" which gave him a mechanical body as we know him of today. He is a notorious jedi-killer with aggressive commanding skills.
When the CIS was being formed, Grievous was kept a secret. He was given command of the invisible hand and a small droid army. He participated in the First battle of Naboo and succesfully won. He, a ordered, began building up his fleet and army to strike when requested.
Early Clone Wars
A year later, the republic had found their clone army. They began dispatcing garrison troops everywhere. Grievous, thinking that the time was right, launched a surprise attack on the Rishi moon outpost. The attack was succesful, and even endured a counter-offensive. During the counter-offensive, Grievous captured clone Commander Bly. He extracted all sorts of useful information and insisted on getting him on the separatist side. It eventually worked, and Bly agreed to join him in exchange for freeing the clones from their republic enslavers.
The Republic, still unaware of the status of Rishi, were unsuspecting when Bly returned and said that everything was fine. Bly and a two commando droids were ordered to infiltrate Kamino and switch our the clone DNA. The operation was a success, and the DNA was switched. The new generations of clones would no longer be loyal to the republic, but the kaminoans were too late to discover that.
Together with the malevolence and STCD, grievous launched an invasion of Kamino. Republic troops were outnumbered, and Kamino fell rapidly. Grievous ordered the violent destruction of all cloning facilities except for Tipoca City. Kamino had been conquered, and the day was won.
He and Doctor Nuvo Vindi started a clone breeding project, with now an army loyal to the themselves. Grievous realized all to well the importance of Kamino, and decided to establish himself there. He and his fleet made several bases on Kamino and almost always blockade the planet.
The war continues
Grievous kept enhancing and enlarging his fleet until he met with a peculiar person. A man called Matt Ordo came to him, saying that he could change the tide of war and bring the republic on its knees. This man had an exceptional amount of knowledge about Count Dooku and Lord Sidious, what on the one side concerned him, but on the other intrigued him. Grievous eventually took him to Dooku, who told Matt that he needed to see results first before joining him. Grievous together with Matt invaded the planet of Mandalore, and forced Duchess Satine to flee.
Grievous was impressed as he saw the large amounts of trained warriors who worked for him. The most notable were Ka Ordo, Yugo Ordo and Darth Maul. Grievous launched a couple of other offensives, capturing Corelia, Raxus-Prime and Rothana. The war was developing well, until an unfortunate matter happend. Clone Commander Bly orchestrated a mass clone breakout from Kamino. Grievous was ordered to destroy every single transport that went out. Bly confronted Grievous in the command centre, saying that the clones would live to fight another day. Respecting his act of sacrifice, Grievous killed Bly and 'per accident' reacted slowly on activating the AA guns, saving several transports from destruction.
Destroying approximately 90% of the transports, the clones got off with a minor victory. Grievous expected to follow them immediattely and wipe them out, but that wasn't the case. Voe Atell pleaded for neutrality with the Clone rebellion, and demanded that no CIS commander were to engage them. Not much after, Voe had fled to the clone base on Alderaan and was banned from CIS space. Grievous still didn't engage the clones on Alderaan, since he knew that it would be a costly battle.
After what appeared to be a republic attack on the clones, Grievous was contacted by Boss, Bly's first lieutenant. He asked the general to donate gear and weaponry in exchange for neutrality. Grievous, of course knowing that the clones were severely weakened, continued to prolong the holocall to trace the location of the call. It was traced to Yavin 4, where Grievous and his fleet then mobilized to. Grievous blockaded the planet, preventing escape. He ordered Boss and Voe Atell to surrender and then the troops would be spared.
However, Count Dooku thought otherwise. He instead ordered the clones to be killed, which was against Grievous' promise. After a struggle, Grievous finally gave in and ordered the attack, much to his distaste. Boss later threatened Dooku to release all the information, as there were clone informants everywhere in the galaxy.
Dooku, believing this was just bluff, 'repogrammed' all clones and sent Boss to Kamino to take part in a clone project. Grievous didn't care, he just carried on with his war effort. However, he soon had to care, as all the information was released and Dooku was imprisoned. Voe Atell had been reinstated as Prime-Minister and now took orders from her. Grievous liked the idea of Dooku for once being lower ranked than him.
Grievous had a feeling that the new mandalorian government was not to be trusted, so he traveled there several times to keep up with things. He was astonished by the amount of quarrel and annoyances there were. Boss, being reprogrammed to an assassin, came to Mandalore to follow grievous' orders. However, he was pre-emptively killed by Darth Maul, much to the general's distaste. The clone rebellion had formed up a clan at mandalore, with Alor, leading them. At first, the general hated Alor for his rudeness and was intent on killing him. However, as time progressed, the general saw some opportunity in the clone commander.
Grievous also befriended Ka Ordo at Mandalore, and supported her when she became a senator. Grievous was also present during the Republic inspection of Mandalore, which was a diplomatic mission under the command of Senator Bail Organa and Obi-Wan Kenobi. Many other events occured at Mandalore, including the switch of power from Matt Ordo to Lord Wrath.
On a certain point, Grievous recieved a message from a mysterious man, who told him to invade a specific republic planet. This mysterious man told Grievous that he worked for Ka Ordo, the senator from Mandalore. Grievous quickly undertook an investigation, and began to distrust Ka since she said she knew nothing. He and Alor ultimately discovered that the mysterious man was Brodo Thul, a befamed bounty hunter.
General Grievous has very strong lightsaber-combat capabilities, and uses the most unorthodox methods to slay Jedi. His mechanical body also makes him resistant to all sorts of damage, including gas and fall damage. Grievous has the largest separatist fleet as of now, and so undoubtedly is a fierce commander to face in battle.