Shadow Realm
Shadow Realm is the Darts combat mode for permanent gold multipliers, Magician Mousey synergy, poison stacks, arcane damage, and Well Rested marks.
The Shadow Realm unlock is a Darts upgrade. Entering starts a timed visit; defeating the skeleton grants a permanent gold multiplier and unlocks the Shadow Mode achievement.
Fire Darts feed Magician Mousey ranks. Golden, Arcane, and Poison darts add gold, boss damage, and poison stack scaling. Well Rested gives one mark every five minutes you stay out of Shadow Realm, then spends those marks as a damage boost when you enter.