Flaccus was a jailer working in the prisons in the Colosseum in 120. He was a tall man who was cruel and sarcastic. He took the Tenth Doctor's pouch containing his sonic screwdriver.
He tricked the Doctor into thinking he was going to be released, when in reality he and Thermus were preparing to shove him into the arena. He believed the prisoners should be grateful for the way they were treated by him and Thermus despite the cruelty and mistreatment, and told the Doctor he should be thankful.
During the revolt and escape of the prisoners, he was sent out with other guards to try to put the riot down. He stayed back, cowering, whilst the other guards were slaughtered by the sheer number and rage of the prisoners. Attempting to back out of the arena when the group turned to him and Thermus, he tripped over a lion that the Doctor had previously put to sleep. This woke the lion up, and he was killed by the beast along with Thermus. (PROSE: The Stone Rose)