Inspired from Halloween (haunted) items in the Valve game Team Fortress 2. Which are only function during the Halloween Season and Full Moons.
Maybe for Terror weapons to be available after the event, every Full Moon (for that 24 hours only? maybe 42 hours for timezones?) allow Terror Annointed weapons and Bloody Harvest Legendaries to drop. Maybe also allow Haunted Enemies to also spawn in?
And future Halloween Events, allow them to also be available on Full Moons, maybe it cycles between them, after the years.
Maybe the concept could be worked on a bit more. But it’d be nice to have the weapons available again after the event ends, but not “always there” of course.