King's Lost Things

According to the witch Excellinor and Old Wrinkly, the heir to Grimbeard the Ghastly will be known by the King's Lost Things:

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.

Hiccup and the King's Lost Things
The fang-free dragon is, of course, Toothless, acquired in How to Train Your Dragon.

Grimbeard's second-best sword is the sword Endeavor, also known as the Dragon Sword, acquired in How to Be a Pirate.

The Roman shield is acquired 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 ruby heart's stone was half of Valhallarama's love promise to Hotshot the Hero, and was given to Hiccup in How to Twist a Dragon's Tale.

The key-that-opens-all-locks was originally owned by Norbert the Nutjob, but was eaten by Toothless. 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 acquired by 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 back to Berk by Humongously Hotshot for Hiccup in gratitude for his actions.

The crown was found by Hiccup under the orders of the witch Excellinor in How to Steal a Dragon's Sword.

The Dragon Jewel was found by Hiccup under the Amber Slavelands in How to Seize a Dragon's Jewel.