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The Morass was an alternate dimension that resembled a red vortex, and was where the Manus Maleficus originated from. According to the Tenth Doctor, time and space in the Morass had been torn apart, leaving only the Reapers and the Chronovores.
When the Crimson Hand used the Manus Maleficus to greatly alter the universe in their image, the drastic alterations to reality caused the Morass to begin to consume the universe, and to threaten to consume all of reality. After Majenta Pryce used the Manus Maleficus to destroy the Crimson Hand and return the universe to normal, the Manus returned to the Morass. The Tenth Doctor briefly entered the Morass with the Manus, before he used it to save Majenta. (COMIC: The Crimson Hand)