CastleStorm

For this achievement, you need to complete the first set of stages and unlock the “Dark Forest” chapter of Campaign mode and Complete the bonus objective on lord of the arena. That includes Ranger Management, Melee Mastery, and Lord of the Arena in the Royal Arena Campaign. You may have to do the bonus objective of Lord of the Arena for the achievement to pop.

Complete the first set of stages to unlock the “Dark Forest” chapter of Campaign mode.