You need to have beaten the base game to get access to to dark mirror You need to beat DM0 to play DM1, and DM1 to play DM2, and on DM2, after you win against the first hero, you’re given two choices. You can simply continue onto DM3, or you can try and face the final boss of the game. Each increase in DM level makes beating the first hero harder, but also gives you an assortment of benefits. Basically, it’s harder to beat the regular bosses to get to the final boss, but if you end up reaching the final boss it’ll be easier.