The Dungeon entrance is actually just a one-floor psychic pitcher plant. The Orb traps the victim in the mental illusion of clearing floor after floor, while his soul is slowly digested to facilitate the Orb's crystalline growth. Like a kinetically-powered watch, the Orb needs the victim to expend mental energy towards an illusory goal in order to capture his vital essence.
Occasionally the Dungeon mimic releases a victim with "loot" from prior victims in order to attract additional adventurers. Eventually the local community begins to catch on, so Dungeon mimics are adept at teleporting to a new location when threatened. The portal, shop and altar mimics one meets in the Dungeon are actually its young, which participate in the illusion to steal sips of energy for their own maturation, and to hone their skills in deception.
The Dungeon exit is a tunnel with a light at the end of it, which is the Orb's means of tricking the victim into "going into the light" - thereby surrendering his life, assuming he hasn't already "died" in combat. The Orb has no natural weapons, and must convince the victim to accept his death, before his faculties deteriorate to the point that he becomes immune to complex illusions. Otherwise he may succeed in escaping while wandering randomly in a zombified stupor.
This is why Dungeon Mimics prey only on sapient species - sentient animals cannot be tempted to remain against their instincts. Dungeon Mimics fear discovery, and therefore reveal themselves only when they sense suitably greedy and credulous candidates. Although their crystalline memory permits them to construct perfect illusions of anything they have encountered, Mimics are merely creatures of instinct, with no capacity to understand complex humanoid motivations. Skeptical minds which insist on remembering WHY they are in the Dungeon quickly break the Mimic's grip. Fortunately for Mimics, the vast majority of anthropomorphic sapients are thinkers in name only.