Sludge

"Say your good-byes, mouse! I won't let you go around telling everyone what I really look like, not when I am about to take a human form thanks to the power of Sproutness! But now, look me in the eyes..."

- Sludge when he captures Geronimo Stilton

Sludge is the main villain of Geronimo Stilton's The Volcano of Fire: The Fifth Adventure in the Kingdom of Fantasy.

The Volcano of Fire
Sludge is a conniving monster who is feared for taking the shape of people's fears. When people show courage towards him does he reveal his true form, which is a colossus of shapeless mud with eyes the color of fire. There is only one thing Sludge is afraid of - water, because it can melt him.Sludge's evil plan is to capture Sproutness, Princess of the Fairies, so he can gain human form.

On his fifth journey to the Kingdom of Fantasy, Geronimo Stilton discovers an earthquake that destroys the Kingdom of the Elves. Aided by Thunderhorn, King of the Elves, Vol, a talking book, and Sterling, Princess of the Silver Dragons, Geronimo goes on a journey to rescue Sproutness, who has been kidnapped.

After a dangerous trek around the Volcano of Fire and encountering the fearsome yet kind pirate Captain Coldheart, Geronimo and his friends discover in the Kingdom of the Blue Unicorns that a race of dwarves called the Diggerts are being forced into slavery in their mines.

The friends bid the friendly unicorns farewell and go to the Kingdom of the Diggerts, where their king, Tyler Terrain III, tells them that Sludge and his scaly henchmen have abducted Princess Sproutness and enslaved the women and children of his kingdom, forcing the men to work in the mines.

Disguised as Diggerts, Geronimo, Vol, Thunderhorn, and Sterling enter the mine and discover Sludge's throne room, where Sproutness is chained up and the Diggert women and children are held prisoner in a giant cage.

Sludge takes Geronimo by surprise in the form of a cat, but Geronimo tells him he is not afraid. Unfortunately, when Sludge turns into his true form, he forces our hero to look into his eyes, making him fall into a deep sleep.

Geronimo is chained up with Sproutness. Sludge tells him his plan to steal Sproutness's fairy powers and weaken them with stinkstone so he can take a human form and rule the Kingdom of Fantasy.

Suddenly, a torrent of water from the River of Oblivion spills into the throne room. Sludge tries to escape, but the water melts his body and carries him into the depths of a deep gorge. Geronimo finally rescues Sproutness and the Kingdom of Fantasy, and with Sludge finally defeated, the Diggerts are freed and celebrate, despite the river flooding their caverns. Geronimo gives a magical medallion to Sproutness, leading her to regain her strength.

Trivia

 * Sludge is one out of a few Geronimo Stilton villains to be a Complete Monster.
 * His personality is reminiscent of Hexxus and Mulgarath.
 * Sludge is probably one of the most evil of the Geronimo Stilton villains, alongside Queen Cackle.