Deadly Shadows are dragons with many specialized abilities prized by the Vikings.

The Complete Book of Dragons presents contrary classification information. In the Dragon Key at the back of the book Deadly Shadows are listed as Sky Dragons. However, in the chapter concerning Mountain Dragons:

"Fishlegs' riding dragon, the Deadly Shadow, is not a Sky Dragon at all but a Mountain Dragon."
The Complete Book of Dragons

Fishleg's Deadly Shadow has a name for each of its three heads: Arrogance, Innocence, and Patience.

To note: There is some contention as to whether the Triple-Header Rageblast stat 'card' was intended to be for the Deadly Shadow. Though the Deadly Shadow features prominently in the Book Series, there is no informational page for it as there are for many other dragon species.

Traits and Abilities

The first obvious trait noticed about the Deadly Shadow is its three heads. Each head is capable of independent thought and Dragonese speech. Each head can produce both fire and lightening bolts from their fire holes. Deadly Shadows have excellent vision from all six eyes, and even has X-ray vision. Each head also has poison sacs, though being poisonous is not mentioned anywhere as being another ability of the Deadly Shadow.

"The poison ducts in his six cheeks pulsed yellow for a second. Out of their secret hiding places crept bright, slitting talons that shone for a second and then slid back."
How to Seize a Dragon's Jewel

This quote from How to Seize a Dragon's Jewel also tells us that Deadly Shadows have retractable claws.

The Deadly Shadow can grow to be ten feet tall and 30 feet long. Their hides can change color to match their surroundings with such precision that they are effectively invisible. They are able to control this ability at will. Their natural colors are shades of sea-green.

Another ability mentioned in How to Betray a Dragon's Hero" of Deadly Shadows is a calming effect on their riders.

"Luckily, Deadly Shadows give off a calming hormone in their smoke when they are in an action situation, and breathing in the milky clouds of steam drifting out of the three heads of the gentle giant beneath him gave Fishlegs the strength to remain calm and to focus."
How to Betray a Dragon's Hero

In the Books

The Deadly Shadow named Arrogance, Innocence, and Patience is introduced in How to Seize a Dragon's Jewel, and features in each of the following books to the end of the series. This Deadly Shadow befriended and remained loyal to a woman named Termagant. Termagant gave birth to a baby boy who was a runt, and by the laws of her tribe, the baby was pushed out to sea. Termagant tasked her Deadly Shadow to follow her baby and bring him to safety. The dragon lost track of the baby and returned with sad news. Termagant had already passed away from illness and grief. The Deadly Shadow looked for the baby but was not able to locate him. Arrogance, Innocence, and Patience became embittered by humanity that led to the death of Termagant.

Years later, the Deadly Shadow joined Furious and the Dragon Rebellion. Furious tasked the Deadly Shadow with hunting down and killing Hiccup. When the dragon finally caught Hiccup, he he did not kill him because Hiccup was wearing the lobster claw necklace Termagant had given to her baby before he floated out to sea. Hiccup got this lobster claw necklace from Fishlegs, who washed upon the shore of Berk as a baby and had no family. Ultimately the Deadly Shadow is reunited with Fishlegs - Termagant's baby boy. Arrogance, Patience, and Innocence now serve Fishlegs faithfully, fulfilling his promise to Termagant.