Answer: Utapau
Utapau was a barren world with some rather bland, brown surface features (it looked like a pale green from space). The planet was actually riddled with large sinkholes, which the natives built their cities in, to protect themselves from the harsh winds on the surface. There were two native races to Utapau, the Pau'ans, the tall, thin, grey-skinned aliens that oversaw the politics of Utapau, while the short and stocky Utai were the labor force.
Grievous attempted to flee the Battle of Coruscant, but was tracked by two Null Commandos (Jaing and Kom'rk) to Utapau. Grievous took over the city and intimidated the locals into not calling for help. Obi-Wan Kenobi headed for Utapau with Commander Cody's forces behind him. The plan was to assault the city if/when Kenobi found Grievous. Obi-Wan did find the droid general, and a lightsaber fight/chase ensued. Grievous attempted to escape in his personal starfighter, but was killed by Obi-Wan on the private landing pad. Obi-Wan would later return here and use Grievous' personal ship to escape Utapau after Order 66 was given.
There were only six Null commandos in the Grand Army of the Republic. They were the original test clones that came before the Alpha ARC commandos. The Kamonians had intended to terminate the Nulls, as they were a 'failure' in the eyes of the Kaminoans, who only valued perfection. Kal Skirata, a Mandolorian, stepped in and prevented their termination, and took it upon himself to train them. They were said to be even more unpredictable than the normal ARCs.