The assassination was led by Ramsay Bolton, the legitimized bastard son of Lord Roose Bolton. Ramsay was approached by Lucius who offered him House Dragen's fealty to House Bolton as well as Ethan's wife, Arrena Storm. However, Lucius's plan was to kill Ethan, give Arrena to Ramsay, kill him, and then march on to The Dreadfort masquerading as Ramsay. This would distract the Bolton forces situated at Winterfell. Lucius would then march upon Winterfell and seize it, ultimately proclaiming himself as the new Blooded King. However, after the Dragens heard about how Ethan died, a small civil war had broken out within the house. This would stall Lucius' plan unfortunately.
Lucius, who already made the deal with Ramsay Bolton, ordered Ethan to go to Winterfell to stop the advance of the Boltons. Lucius gave Ethan 30 soldiers who were, unbeknownst to him, secret assassins ordered to kill him. Ethan confronted Ramsay, planning to go to battle with him. Ramsay, however, kept insulting him about his bastard status, claiming that even his own brother didn't bother to legitimize him. This angered Ethan somewhat, which, in turn, distracted him.
Ramsay nods at one of Ethan's men, which apparently is the signal for the assassins to attack. His close friend, Josmus Snow, who was also involved in the conspiracy grabs Ethan and pushes him around, yelling at Ethan's men to attack him. One of the assassins charges at Ethan with a knife. Ethan grabs a hold of the knife, cutting his hand in the process. All the assassins charge at Ethan all at once and stab him from all directions while Ramsay sadistically smiles at Ethan's gruesome stabbing. Ethan drops to the ground, bleeding profusely. Ramsay gives Josmus a dagger and whispers in his ear to "finish it". Josmus slowly walks over to Ethan, visibly mortified. Ethan stares him directly in his eyes and whispers: "You too, Josmus?" Josmus deals the killing blow by driving Ramsay's dagger into his heart.