Carceri is a well-hidden astral dominion. Its entrance is barred by great gates of adamantine. Those who breach the red color veil of the dominion are deposited in front of those adamantine gates. They cannot enter the dominion further without discerning a way to open or bypass the gates.
Carceri is a prison plane. The deities created Carceri as a place to imprison a terrible primordial monster that was too powerful to destroy without unleashing power sufficient to destroy all existence. Moradin himself forged the mighty gates and the chains that bind the monster inside.
Carceri takes the form of a bleak archipelago beneath a starless sky. Six barren islands lie beyond the adamantine gate: Othrys, Cathrys, Minethys, Colothys, Porphatyx, and Agathys. The first five islands are cold and marshy and sometimes lashed by acidic rain. They are inhabited by a wretched, law-less handful of stranded creatures—mortals, mortal spirits, devils, and monsters of all sorts—who were sentenced to perpetual imprisonment in this plane. The strongest among these prisoners and outcasts dominate and enslave the weak.