Season 12 delivered the fourth class change — new skill trees, quests and a renewed master system — plus Nixies Lake, the Labyrinth of Dimensions and the Deep Dungeon. It is the single biggest progression overhaul between Season 6 and Season 19.
A dedicated quest line unlocked the 4th class for every character, complete with new skill trees and a rebuilt master skill system. Power curves, builds and server metas all reset around this moment.
A frozen lake hunting ground with its own boss zone, tuned for the newly empowered 4th-class characters and stocked with Holy Angel / Dark Angel-era rewards.
An instanced, floor-based challenge dungeon with weekly rankings — MU's first serious instanced PvE loop, later refined in Season 16.
Part 2 added the five-floor Deep Dungeon for high-end grinding, while Holy Angel armor and Dark Angel weapons extended the mastery ladder another rung. A jewel-bingo minigame rounded out the fun layer.