It spawns after killing about 4 series of "A sludge lurker" spawning at the middle of pyramid over NW corner surrounded by earth golems. Each time killing "A sludge lurker", will spawn many low lvl adds. After clearing the adds, will have next sludge lurker spawned at the same point. On the last sludge lurker killing, there will be 5 "A Filth Gorger" spawning at the pyramid and rush to where u killed the sludge lurker. After killing the 5 Filth Gorger, A Fabled Monstrous Mudwalker spawns and rushes into camp.
It hits around 5600 max and casts ae root. During fight there will be low lvl adds spawn too. Prepared for that and Goodluck.