After the Clone Wars officially kicked off with the Jedi facing down a horde of droid enemies, General Grievous was actually lurking menacingly in the tunnels of the Geonosis arena. The General had freshly received his cybernetics from count dooku and was instructed to safeguard the Separatist council from any Jedi threat, but was told to avoid being found at any cost. This meant he had to kill any Jedi who laid eyes upon him.
Following this, General Grievous was let loose into the dark and cold catacombs of Geonosis, waiting creepily to stalk and kill any jedi who dared approach him. Unfortunately for one Jedi, he accidently stumbled into the dark catacombs during the battle and was swiftly torn to shreds Near the end of the battle, a large group of Jedi attempted to flee into the catacombs to escape the impending droid threat, but again they ran into Grievous who slaughtered every single one. The Jedi didn’t discover Grievous’ involvement at the battle until much later, leading them to realize the strings of the war had been in motion for much longer than they realized.