Hey there. Unfortunately the wording for these seasonal challenges is always slightly ambiguous. The 'seasonal' weapons it asks for are the weapons from Nightmare Containment and Sever: Hollow Denial, Bump in the Night, Tears of Contrition, Nezarec's Whisper, Firefright, Without Remorse. It is not enough to just have a Season of the Haunted icon on the weapon. Trespasser, Chain of Command, the dungeon weapons, the reprised Opulence/Menagerie weapons, random world drop weapons like Enyo-D and Lunalata-4B, are all weapons that do not count for this seasonal triumph.