Tanniel was a military officer who served the Galactic Republic and the Galactic Empire. During the Clone Wars, he was appointed Moff of the Quelii Oversector under the Sector Governance Decree and assumed the role of commander of the 10th Sector Army. When Emperor Palpatine declared his Galactic Empire in 19 BBY, Tanniel became one of the first twenty Grand Moffs. He later oversaw development of the Imperial Storm commandos. Tanniel placed Crix Madine in charge of the first unit of Storm Commandos deployed during the Galactic Civil War.