The Mud Kingdom is the home of the MudWings, who thrive in the swampy, humid climate. It is on the back of Pyrrhia under the Sky Kingdom's wing and next to the Rainforest Kingdom and the Kingdom of the Sea. It was described by Turtle as "dreary, swampy, and bog-riddled."
The Mud Kingdom is a territory in eastern Pyrrhia, north of the Rainforest Kingdom. Queen Moorhen's Palace is next to a large lake at the southern edge of the Mud Kingdom, around a day's flight from the rainforest. At least two scavenger dens are known to be within the Mud Kingdom's territory. The Diamond Spray Delta is known for being the home of the lowest-born MudWings. There is a path of dried dirt that runs through the Delta's village. On either side, there are large mud puddles for MudWings to sleep and hunt in, as well as sleephouses. Sleephouses are tall mud towers that each MudWing troop of sibs sleep in. Generally, one troop sleeps in a certain sleephouse. The rest of the Kingdom is unexplored but it's presumably swamp and marsh lands.
Clay meets his mother, Cattail, along with his siblings Reed, Pheasant, Umber, and Sora with Glory near the Diamond Spray Delta, which, according to Kestrel, is where "the lowest-born MudWings crawl". The dragons of the Mud Kingdom live in huts with their sibling units made of mud with holes in them to enter and exit.
Glory, the dragonets, and Webs walk along the outskirts of the Mud Kingdom to get to the rainforest. Starflight claims Queen Moorhen's palace is near a large lake and only a day's flight away. They also find two dead MudWing soldiers, killed by the NightWings.
Sunny and Clay went to the Mud Kingdom so they could visit the dreams of the MudWings with a dreamvisitor, to convince Burn into coming to the meeting. They learned that the MudWings were planning an assault on the IceWings, and managed to convince Queen Moorhen to drop the idea.