Viggo's Dagger is a weapon used by Viggo Grimborn in different circumstances.
Viggo's dagger resembles one of Eret's Swords, but it is shorter and blunter, thus making it not a very effective weapon.
While going with Hiccup to stop Ryker in "Shell Shocked, Part 2", Viggo tells Toothless to stop in the middle of the ocean. He then takes out his dagger, appearing as if he wanted to kill Hiccup. Before he could do anything, Astrid tells him to stop, making Hiccup aware of Viggo's action. He then says that he wanter to drop it all the time and lets it fall into the water. A Submaripper submerges, revealing Viggo's plan.
Strangely, Viggo manages to get his dagger back, as he is seen using it again in "Triple Cross". When he, Hiccup and Toothless went after a Skrill, Viggo took out his dagger and showed it to the wild dragon, explaining how the Berserker Tribe used to lure Skrills into their traps, using metal daggers, as the dragons are attracted to them due to their electrical properties. He manages to tame the Skrill that way.