Blister

"Lief had seen enough blister victims fall, writhing in agony, to know that he did not want to risk the same fate."

- The Forests of Silence. A blister is the colloquial name given by the people of Del for a silver egg filled with Ooze Toad poison, used by Grey Guards particularly during the Shadowlands invasion. They burst on contact and burn the victim, resulting in a quick and agonising death. Each guard carried a leather sling for throwing and a supply of the eggs. They were able to throw them with immense strength and accuracy.

History
During King Alton's reign, the Shadow Lord gained a servant in the Ooze Toad Gellick and created the blister to arm its Grey Guards with projectile versions of its deadly poison. After Gellick subjugated the Dread Gnomes, it forced them to leave large quantities of the poison they harvested by the roadside at the base of the mountain each full moon. Once the Dread Gnomes left, pods of Grey Guards would approach and collect the poison for their weapons. The jars of poison were then carried through a pass to the Shadowlands factory, where they were used to make the blisters.

The Forests of Silence
Lief reflected on blisters while running away from two Grey Guards in Del after her stayed out past curfew. The memory of past blister victims dying in agony kept him moving.

The Lake of Tears
During the rescue of Manus from a pod of Grey Guards, the pod chased Lief and Barda through a forested area. The two quickly realised that as soon as they reached an open path, blisters would start being thrown.

The Shifting Sands
After two Grey Guards who had been pursuing Lief, Barda, and Jasmine into the Shifting Sands were killed by a Sand Beast, the companions collected their blisters. Barda later slung two blisters at a large Sand Beast that tried to attack Lief, but the creature was only held back by the poison and not killed. However, this allowed Lief and Jasmine to escape.

Dread Mountain
Lief, Barda, and Jasmine collected blisters from two more Grey Guards after they were transformed by the water of the Dreaming Spring. Later, when cornered by the Dread Gnomes, Lief offered to free them from Gellick's tyranny by using blisters on the creature. However, since the blisters were made from the Ooze Toad's own poison, they had no effect on him.

After Gellick's transformation into a tree through Dreaming Water, the Dread Gnomes created a false version of his poison by mixing Boolong Sap with water. They continued to supply the Grey Guards with the fake poison each full moon and kept what was left for themselves, knowing one day that the Shadow Lord would catch onto their deception and attack.

Return to Del
In Del, Steven tried to calm Nevets, warning him against attack due to the Grey Guards' blisters and daggers.

Running to save Dain from burning, Lief, Jasmine and Barda dodged every blister that flew at them.

Guards who stood at the Place's edge were tasked with throwing blisters into the crowd at any sign of disobedience. Lief speculated they would have their slings at ready, with an abundant supply of blisters in boxes behind them.

In the confusion after Lief put on the Belt of Deltora, revealing himself to be the heir and causing the Shadow Lord to retreat, the remaining Grey Guards tore blisters by the dozens from the boxes, throwing them at the celebrating crowd. However, the people of Del had already learned that water and Boolong Sap was not harmful.

The Shadowlands
Jasmine remembered that poison for the blisters had been carried through a pass that led from Dread Mountain into the Shadowlands. She theorised that the Shadowlands factory had to be near the pass, on the Shadowlands side, as no one would move jars of deadly poison any further than necessary.

Trivia

 * In the anime, the blisters act similar to hand grenades. Instead of just shattering and coating their victims in venom, they create small explosions and can cause burns, as was shown when Jasmine set fire to a Hive swarm. This is inconsistent with Gellick's poison in the anime, which is shown to turn its victims into stone when spat by Gellick, yet acts like it does in the book when coated onto the Dread Gnome's arrows.
 * In the anime, they are launched with gauntlets rather than hurled with slings.