Tarr Seirr was a Cerean Jedi Knight of the Old Republic who served as the watchman of his home system in the years leading up to the Clone Wars. While the rate of male births on Cerea was low, it seemed that the rate of Force-sensitivity in males was noticeably high. Tarr Seirr represented the Cerean race alongside such famous Cerean Jedi as Ki-Adi-Mundi and Tap-Nar-Pal. As a young Jedi, the position of watchman of the Cerean system was held by Ki-Adi-Mundi for a number of decades and Tarr Seirr revered the Jedi Master. As the Separatist crisis gained momentum in the years leading up to the Clone Wars, Ki-Adi-Mundi was called to other tasks and gave up his role as watchman to Tarr Seirr.
Following in the footsteps of the Jedi he idolized, Tarr Seirr worked closely with the Cerean government to weather the Separatist movement. While Cerea was not an official member of the Republic, it did maintain a close relationship with the Republic, and Tarr Seirr worked hard to avoid having Cerea drawn into the movement. When the crisis resulted in war, many Jedi watchmen were recalled in order to be reassigned to the war operation. While Tarr Seirr regretted leaving his home system, he knew the tenets of the Jedi Order required detachment from outside influences, even if it was his own homeworld.
During the Clone Wars, Tarr Seirr was assigned to work with Master Ki-Adi-Mundi. Four months after the Battle of Geonosis, Jedi Master Daakman Barrek discovered a secret Separatist operation on the planet of Hypori. Massive droid factories were built in an effort to churn out thousands of battle droids and super battle droids to increase the size of the Separatist armies. The operation was commanded by General Grievous, a new cyborg commander discovered by Count Dooku and trained in the Jedi art of lightsaber combat.