The Dead Swamp, the length of Waterfall once known as the Echoflower Swamp, has suffered greatly from the blocking of its source with trash and dust, along with the fading of the magic of the glowing crystals, both the unmoving current and the lack of light turning the waters murky, grey, and rank. The smell of death permeates the caverns, hanging heavy in the air, and nearly all of the bridges and paths have been swallowed by the swelling, fetid waters, dusty mud, abandoned, empty clothes, garbage, and chewed bones cluttering the waste of once beautiful marshland.

The echoflowers have become likewise twisted, mutating into carnivorous plants that whisper pleas for help in the voices of those long passed, drawing in helpless prey to be devoured. The mosses, mushrooms, and various other flora have changed as well, altered to spray toxic spores and trap anyone who steps onto them.

Through the mud caked paths and walkways roam the Temmie horde, crying out into the darkness and rolling in a wave of bodies through the muck. Not even Undyne dares stand in their path.