The Marsh is the first dungeon area in the game, located in Lakeview. It becomes accessible at levels 1-5, and the mutated version at levels 51-55. It is followed by the Sewers in Junkton.
The area boss for this dungeon is the Lake Monster, and in the mutated version you may encounter the Lake Demon possessed boss.
The Kitten ally can be found in this area, and always will be during the tutorial.