Grey tide

"This was a monster they could not fight. The grey tide would continue to spread. It would swallow rivers, forests and plains. It would cover towns and villages and farms. It would fill the valleys and smooth out the hills."

- The Sister of the South

The grey tide was a thick, poisonous, grey liquid and the last and most powerful of the Shadow Lord's projects. It was created to engulf the land of Deltora if the Four Sisters were destroyed, but was itself destroyed by King Lief and the seven remaining dragons.

History
After the Shadow Lord's defeat at the Battle for Deltora, it decided on a long term plan to invade the land by weakening the people's trust in their king. It also developed several contingency plans in case its main one failed. These plans developed into the Four Sisters, creatures based off of the Shadow Lord's encounter with four sisters that sung to keep a great beast from destroying their island. Their purpose was to starve the land to make it easier for invasion, but should they be destroyed, the Shadow Lord also created the grey tide, which would destroy all life in Deltora.

Knowing that Deltora's dragons would destroy its creations, the Shadow Lord sent its seven Ak-Baba to exterminate them. By the time of King Lucan, their numbers were small enough for the Shadow Lord to begin its plans.

Secrets of Deltora
Once the dragon population was nearly eradicated, the Shadow Lord had its servants begin to place its devices around Deltora. Chief Advisor Drumm oversaw the placement of the Four Sisters, while the Shadow Lord had nine others breed a rat plague to force the people of Hira out of their city. Once it was abandoned, the Shadow Lord had the grey tide placed among the ruins.

The Sister of the South
While Lief, Barda, and Jasmine were destroying the Four Sisters, palace librarian Josef learned of the existence of the grey tide. He tried to warn Lief, but his mind and letters were tampered with by his assistant Paff, who was a servant of the Shadow Lord. Eventually Paff killed him with poison, but Josef was still able to warn Lief by letting him read the tale of the Four Sisters in his book, Tales of Deltora.

After Lief and the last Topaz dragon destroyed the Sister of the South, Lief had a brief vision of the grey tide when he used the power of the Opal to restore Barda's sight. He learned of its existence through Josef's book, and left with Barda and Jasmine on Fidelis to the city of Hira. There they spotted a huge yellow bubble emerging from the city that would eventually burst oozing the grey tide. Hopian, the last of the Opal dragons, was awakened, and after Lief calmed his rage, he and Fidelis combined their power to burn the tide as it spread across the plains. However, their attention was diverted when the Shadow Lord used powerful magic to defy the Belt of Deltora and send its seven Ak-Baba to battle the dragons. Knowing they were outmatched, Lief summoned the other five dragons to help them.

Hopian was able to tear the throat out of one Ak-Baba, and Jasmine killed a second when it attacked the Opal dragon, but it was evident that they would be overrun. However, the other five dragons arrived and killed two more Ak-Baba, forcing the remaining three to flee back to the Shadowlands. The dragons then turned their attention to the grey tide and combined their fire to burn it until nothing remained.

Appearance
When it awoke, the grey tide was a massive orange boil that grew larger than the buildings of Hira.

Abilities
The grey tide was created to destroy Deltora if the Four Sisters were ever destroyed. It does this by producing a thick grey liquid that flows out of its body and covers everything in its path. One touch is enough to kill a living thing, and simply touching water causes nearby fish to die. However, the slime would harden when hit by dragonfire.

Trivia

 * In City of the Rats, the horde of rats that pours out of Hira is described as a "grey tide", in a possible foreshadowing of the grey tide to come.
 * In the tale of the Four Sisters, it is said they kept a giant beast (which lived at the centre of the island) at peace until their singing ended. When the four (evil) Sisters were destroyed, by Deltora's dragons, it unleashed the grey tide. This is a reference to the original beast that was awoken by the Shadow Lord.