"The colossal slug known as Slog presides over its fetid domain of the Cloaca. It bathes its bloated, slimy body in the moiling pools of dross, keeping itself moist and waiting for fresh prey to be churned into its lair. Whereas nature had the decency to end the body of the common slug in a muscular foot, Slog's tail is instead a sphincter-choked bilge hose that it uses to spew at its prey the filth of the Cloaca."
Slog is a big challenge. His ranged damage isn't too bad but in melee he will often deal 20 damage per round. In addition, he seems to move easily through the pools of goop in the Cloaca, and is surprisingly swift on his foot for a giant slug monster. His attacks will Poison, Rust, or even give you Diseases.
Slog's ranged attack requires him to stand in liquid, which he then flings at you to knock you prone. Being knocked prone subjects you to effects dependent on sludge type: Black Ooze causes Glotrot, Green Goo causes Poison, Brown Sludge rusts items. Slog-created pools seem to always include Goo, even if he was standing over different liquids.
Note that fighting Slog is entirely optional (provided you can avoid him.) Sometimes, if you're quick and lucky, you might get through the last floor of Golgotha without encountering him at all.
Having a high relationship with Mollusks faction and performing a Water Ritual with Slog opens an option to learn the unique recipe Cloaca Surprise that will permanently transform the player into a slug-like creature.