Luminous Krayfin

"The Luminous Krayfin thrives in a humid habitat and must feed on the Dragon Bloom, a most dangerous flower. Only then can it become a leviathan. Extremely dangerous. Pray you never see this monster on the seas."

- Bork's Papers

The Luminous Krayfin is a large Tidal class dragon that made its first appearance in the School of Dragons expansion "Secret of the Leviathan". As an adult, it becomes a Class 10 Leviathan, just like the Bewilderbeast.

Egg
The egg of the Krayfin was previously referred to as the "Mysterious Blue Orb". It is a large round egg that is a glowing cyan blue in color with darker blue swirls around its circumference.

The "Mysterious Blue Orb" was first seen in the School of Dragons Expansion "Battle for the Edge". In this expansion, the Dragon Hunters first had it, and later Harald Forkbeard steals it from the Hunters. At this time, the player is not sure what the Orb is.

The "Mysterious Blue Orb" reappears in the next School of Dragons expansion: "Return to Dragon Island". Through various quests, the player and the Dragon Riders gain control of the Orb, which the player realizes is an egg. The expansion ends with the egg hatching and Hiccup saying his famous line, "This changes everything". The following expansion, "Secret of the Leviathan", reveals that the egg has hatched into the Luminous Krayfin.

Hatchling and ShortWing
The Hatchling and Shortwing growth stages of the Krayfin are seen in School of Dragon's expansion, "Secret of the Leviathan". It has four short legs, each ending in a long flipper, and long wings. It is capable of long flight at this age. Its body is rotund and has a chunky short-snouted head. It has a line of luminescent spots trailing along its body. Depending on the lighting within the game, the Krayfin appears to be indigo or dark blue, gray at other times, and even dark green.

According to Hiccup, the Luminous Krayfin grows rapidly.

In the expansion, "Wrath of Stormheart", Fishlegs indicates that a Krayfin's growth is akin to that of a frog, meaning each growth stage physically changes to suit different environments. The hatchling Krayfin is able to live out of water and walk fairly easy, and even fly. However as it matures, the wings elongate and become thinner, more suited for living an entirely aquatic life.

Adult and Leviathan Stage
These growth stages are unknown yet. However, the "Bork Papers" reveal that in order for the Krayfin to become a Leviathan (Tidal Class Titan Wing), it needs to eat a plant called the Dragon Bloom. The dragon bloom produces a special fruit that increases the growth speed of the Krayfin.

By the end of the expansion, the Krayfin has grown tremendously, but the player does not know if this is an adult size or still a larger juvenile size.

Based on the Krayfin skeleton underwater at Vanaheim, Lumie is still very small compared to the enormous size of the skeleton.

Speed and Agility
As seen in The Secret of the Leviathan, the Luminous Krayfin is a very fast swimmer as it was able to swim all the way to Impossible Island in a few minutes after he was scared away.

It is also able to jump from the water and spin in the air, despite its size.

Intelligence
Even as a baby, the Luminous Krayfin was shown to be intelligent, understanding what he can do to help the player through the labyrinth.

Jagged Back
According to the Bork Papers, an adult Luminous Krayfin has a jagged, rocky back that is powerful enough to break a Viking ship in half.

Low-Frequency Sound Production
The Luminous Krayfin is capable of producing low-frequency sounds. This is likely used as a method of communication, as these sounds can easily be heard by other members of the species from miles around.

Bioluminescence
The Luminous Krayfin gets its name from the bioluminescent lights on its body. These lights likely allow it to see in deep water, though they may also allow it to communicate with other Krayfins in the depths of the ocean.

Sense of Smell
A baby Luminous Krayfin is able to smell a Dragon Bloom from far away and track it down, as seen when one was able to find Impossible Island, a place where a Dragon Bloom grows.

Trivia

 * The Luminous Krayfin is the first Franchise dragon to start with the letter "L".
 * The Lycanwing also starts with an "L", but it is unknown if it truly exists
 * The term Leviathan seems to be more of a classification based on the sizes of Tidal Class dragons, not to be confused with the actual name of the dragon itself.
 * This dragon is not a rideable dragon at this point in the game. However, if you leave and return to Impossible Island you will see it swimming in the lagoon at the bottom and if you time a jump from the bridge just right you will land on his back and not be forced to respawn on the bridge.
 * The adult form of the Luminous Krayfin resembles a combination of a seal, a sea turtle, and an alligator snapping turtle.
 * The sounds emitted by the baby Luminous Krayfin are the same as Torch's as a baby.
 * The Luminous Krayfin is similar to a Pokémon called "Lapras". Both of them are rare, big, fictional water creatures first seen in a role-playing game. Also, due to the fact that the Pokémon can use dragon-type moves (such as Dragon Dance and Dragon Pulse) it means both are remotely connected to dragons. To the aspect part, both are blue creatures with a long neck in a swan-like pose, flipper-like limbs, a light underbelly, and a brown shell with bumps on which people can sit (although this feature in School of Dragons may be actually a glitch).