Dagur's crossbow is the weapon that Dagur the Deranged used in his dragon hunting, training on Dragon Island, as he prepared himself for hunting and killing a Night Fury in Defenders of Berk episode "The Night and the Fury".
Dagur first got out his crossbow at his camp sight when he heard sounds and pulled it out to defend himself and possibly Hiccup. Dagur later brought it with him as he brought Hiccup alone for his hunt for a Night Fury; Hiccup pushed it once so the young Berserker Chief wouldn't get the chance to kill a wild Monstrous Nightmare.
Dagur later used it to fight Hiccup, when he learned the truth about Berk's Dragon riding and training; and the "Dragon attack" that he and his men faced on Berk was an act. He even tried to use it to kill Toothless, but failed.