The King's Lost Things are an assortment of 10 items that, according to Excellinor the Witch and Old Wrinkly, can be used to identify the new King of the Wilderwest, the heir to Grimbeard the Ghastly. They are first mentioned by Old Wrinkly in How to Steal a Dragon's Sword when prompting Hiccup to look after them and later confirmed by the witch Excellinor when she is explaining to the Viking tribes the need to elect a new King of the Wilderwest. Many of the items listed were in possession of Hiccup who had gained them over the course of the series.
The Prophecy of the King's Lost Things
|“||The Dragontime is coming, and only a King can save you now. The King shall be the Champion of Champions. You shall know the King by the King’s Lost Things: A fang-free dragon, my second-best sword, my Roman Shield, an arrow-from-the-land-that-does-not-exist, the heart’s stone, the Key-That-Opens-All-Locks, the ticking-thing, the Throne, the Crown. And last and best of all the ten, the Dragon Jewel will save all men. [src]||”|
|— Prophecy of the Wilderwest|
Hiccup and the King's Lost Things
Throughout the book series Hiccup has managed to find many of the King's lost things.
The fang-free dragon is, of course, Toothless, acquired in How to Train Your Dragon. Hiccup picked him up in a lightless tunnel while running from the mob of wakened dragons in Wild Dragon Cliff; he claimed that he'd just known that the sleeping dragon was there, and that he'd felt he was destined to have this dragon.
The Roman shield is acquired when Hiccup accepts the Fat Consul's surrender in How to Speak Dragonese.
The arrow-from-the-land-that-does-not-exist was stuck in the potato on Hysteria in How to Cheat a Dragon's Curse.
The Key-That-Opens-All-Locks was originally owned by Norbert the Nutjob, but was eaten by Toothless during the theft of the potato during How to Cheat a Dragon's Curse. Hiccup found it attached to the magnet in Toothless' belly in How to Break a Dragon's Heart.
The ticking-thing was the watch/compass/navigational instrument that Norbert's father had used to sail to America; it was stolen by Camicazi and given to Hiccup in How to Cheat a Dragon's Curse, and Hiccup used it to navigate to America and back in How to Ride a Dragon's Storm.
The throne was washed up on a beach in How to Break a Dragon's Heart, and brought to Berk by Humongously Hotshot and his new wife for Hiccup in gratitude for his actions.
The crown was found by Hiccup under the orders of Excellinor the Witch in How to Steal a Dragon's Sword. It was being protected by the Wodensfang, who willingly let Hiccup take it.
However when Hiccup is declared to be a slave and Stoick the Vast a traitor by the witch Excellinor, Hiccup is forced to give up eight of his nine things to Alvin the Treacherous, who is then declared as the rightful King of the Wilderwest due to being placed third in the sword fighting competition. The only one of the items remaining in Hiccups possession is the fang free dragon, which refers to Toothless.
The Dragon Jewel was found by Hiccup under the Amber Slavelands in How to Seize a Dragon's Jewel. However, this was also lost to Alvin the Treacherous.
This Jewel was found to be a "red herring", and the actual Jewel was in fact hidden in the Lobster necklace, and only discovered in the twelfth book. Toothless was able to use his hard gums to crack open the lobster claws and reveal the Jewel.
The true Jewel contains a fossilised nanodragon infected with deadly plague. If the Jewel were to be broken, all dragons would die in a pandemic.
In How to Betray a Dragon's Hero, Hiccup attempts to retrieve the King's Lost Things. He temporarily manages to steal them all from Alvin and tries to escape from him using a boat, but before he can escape his boat is torn down and all the Things sink to the bottom of the ocean. The Things are then retrieved by the Alvinsmen and presented to the Druid Guardian, who agrees to crown Alvin.