The Bloody Harvest event in Borderlands 3 is live, there are a lot of challenges to complete, multiple bosses to eliminate, and loot to collect. The new questline has a set number of steps that you have to complete in order to progress. The event has will kick off once you go ahead and talk to Maurice located near the Mayhem switch in Sanctuary.
Everyone is busy completing their quest which starts from searching ghosts in the galaxy and then moving further with eliminating a set of enemies. This set of enemies includes many bosses such as Captain Haunt, El Dragon Jr, and Haunted Borman Nates. You also get the Fearmonger upon taking down the Captain Haunt, this is a pretty good legendary shotgun only available for the duration of the event.
Haunted Demoskaggon Location
To find Haunted Demoskaggon just fast travel to the Droughts on Pandora. His exact location is marked in the image below. Do remember that Demoskaggon is not a guaranteed spawn, so you might have to reload the map a few times. I had to do it three times to make him spawn.