The story of Darkest Dungeon is set in a gothic, Lovecraftian world. The player inherits an ancestral estate that is haunted and corrupted. You recruit various heroes to explore the dungeons beneath the estate which are filled with all sorts of terrors. There are battles against eldritch abominations, and as the heroes face these horrors, they can gain stress and afflictions. The goal is to cleanse the dungeons and restore the estate, but it's a long and arduous journey filled with death and despair.
In the Darkest Dungeon full story, initially, the discovery of the corruption within the estate is a key event. This leads to the recruitment of the first batch of heroes. As they explore, coming across special areas in the dungeons like the Cove or the Warrens are important. These areas have unique enemies and challenges. The hero - affliction system also plays a big role. For example, when a hero goes mad and attacks another hero during a battle, it changes the whole dynamic. And finally, the push to reach the heart of the corruption in the final dungeon is a very significant event in the overall story.
It depends on your perspective. Berserk is definitely dark, with its violent and tragic storylines. But some might argue there are other mangas that could be equally or even darker based on personal preferences.
One of the darkest Marvel comics is 'The Punisher MAX'. It delves deep into the gritty and violent world of Frank Castle with very intense and disturbing storylines.
Well, for a manga to be the darkest, it often features a lot of hopelessness, a cruel and unforgiving world, and characters facing constant and overwhelming hardships. Also, the art style might contribute, with heavy use of shadows and dark colors.
Hellboy has its dark moments, but it's not unanimously regarded as the absolute darkest. The perception of darkness varies from reader to reader, and other comics might push the boundaries further in that aspect.