Three Kingdoms: I Land in a Bandit's Den from the Start

I transmigrated into a military officer in Zhuo Commandery. The moment I opened my eyes, I was locked in a bandit stronghold's water prison, with my younger brother Liu Ping'an beside me, burning with a high fever. The original owner had already frozen to death, but then I heard bandits talking overhead. They said they would use us in a ceremony to raise their battle flag that night. At last, the System chimed in, but it said the condition for unlocking it was to kill a hundred bandits.