The Twilight Forest Swamp biome has Mangroves and swamp oak trees, with vines growing throughout. Many small pools of water have sand or clay at the bottom. Underneath the swamp, there are Labyrinths filled with Minotaurs and other creatures. Mosquito Swarms are very abundant in Twilight Swamps.
If you are playing according to the progression system, the swamp will be filled with an thick haze of mosquitoes. Staying in the area will give you hunger sickness. Defeating the Lich will remove the curse on the swamp.