Serenno was a planet in the Outer Rim Territories, known for the wealth and privelage afforded to its aristocracy. More specifically, the six great houses of Serenno, each of which was headed by a count. The planet had always been a hotbed of anti-Republic sentiments during the Clone Wars, and was the base of operations for Count Dooku who possessed a somptuous palace there. It was defended by a larg separatist fleet. It was primarily covered with forests, and mountainous regions.
Dooku usually remained at his lavish estate, where his former associate Asajj Ventress made an attempt on his life.
Asajj Ventress attempted to kill Dooku on Serenno after he betrayed her in the Battle of Sullust. From that point on, the Count's personal security played a greater role in his planning ultimately making him vulnerable to the Nightsisters. The Sisters had unbeknownst to Dooku assisted Ventress in her failed plot and were responsible for creating the monstrous acolyte Savage Opress who was intended to be his new assassin. The Count trained Savage Oppress there, and Serenno was used as a base by the fleet that attacked Dathomir. Dooku was in his palace on Serenno when Mother Talzin attempted to use her powers to kill him.