The Master Level system is the endgame progression path. After reaching level 400 and completing your 3rd class evolution, you unlock a separate Master Level track with its own skill tree.
Master experience comes from killing monsters, but only monsters at or above a minimum level count. It is not tied to a specific map, so any qualifying monster anywhere grants Master XP.
Icarus, Aida, Kalima 4-7, Land of Trials, Kanturu, Crywolf, Karutan, Vulcanus, Acheron, Deventer, Swamp of Calmness, Raklion, and every map above them. Lower maps (Lorencia through Lost Tower and Tarkan) have monsters below level 95 and grant no Master XP.
Deep Dungeon, Nixies Lake, and the endgame chain (Cubera Mine, Atlans Abyss, Scorched Canyon and beyond), where monster levels climb well past 160.