Darkbreathers are enormous Sea Dragons, much despised by Vikings. They inhabit the Open Ocean and the waters of the Barbaric Archipelago, particularly the Inner Isles. Darkbreathers first appear in the short story, How to Train Your Viking.
Darkbreathers get their name from their cavernous mouth, that is lined with razor sharp, jagged fangs and has a throat inside that appears as a void. They have long snake-like bodies, and wide jaws that can also unhinge like a snake's. In this way they are able to swallow ships whole if they are so inclined. Darkbreathers are also said to have rather small brains. Darkbreathers can speak Dragonese. The Incomplete Book of Dragons indicates they have an excellent sense of smell.
Darkbreathers live on the bottom of the ocean, so far down there is no light. Like many creatures living in darkness, they are attracted to light. Though Darkbreathers come to the surface for an attraction to light, or for hunting, they cannot remain there for long. They are poisoned by too much oxygen, and must return to the anoxic water of the sea floor.
Darkbreathers hunt two different ways. On the bottom of the ocean, they stay quite still with their giant maws wide open and wait for an unsuspecting fish to swim inside, thinking it has found a safe haven. They are rather low energy and prefer to conserve it. Secondly, Darkbreathers swim up to the surface underneath prey and grab them from below. Once they get a hold of their victim, they drag it down into the ocean and suck out its blood, undisturbed.
Darkbreathers are not trainable, and are instead hated by Vikings because they often damage their ships. Vikings use a clever tactic to lure Darkbreathers into the trap. They send a dead dragon carcass afloat and wait for the beast to appear. Once it launches out of the water, the Vikings fire nets and arrows.
The Vikings are warned about Darkbreathers during the Swimming Race. Fishlegs also panics when he can't swim, and is worried there are Darkbreathers in the water. Fishlegs hopes that Snotlout will get eaten by a Darkbreather.
This reference book discusses particulars about the Darkbreather species.