Darkbreathers are dragons that swim up from the bottom of the ocean and snatch their prey. Once they get a hold of their victim, they drag it down into the ocean and suck out its blood. It first appears in the book How to Train Your Viking. They get their name from their cavernous mouth, that is lined with razor sharp teeth and has a throat inside that appears as a void.

They also appear on the map at the beginning of A Hero's Guide To Deadly Dragons.

In How to Ride a Dragon's Storm, 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.

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. Darkbreathers are hated by Vikings because they often damage their ships.