Accursed Spirits are an unofficial name for the spirits of the dead that roam the astral plane and the Interstice. They are among the creatures drawn to the Brand of Sacrifice, and come out only at night, being burned by and afraid of the sunlight.
Personality
Accursed spirits are restless and tormented, clinging to life due to lingering regrets. Despite their somewhat pitiable nature, they are also contemptible and hostile, and will attack and possess whatever they can get their hands on. Spirits that get too close may become aware of dark secrets and thoughts that a person may have been hiding, and will use this to break down the minds of a victim and force them to act on them, as happened with Farnese.
Restless spirits, despite being repelled by sunlight, are described as yearning to possess a living body because it is like a warm flame. Spirits have never been shown actively possessing corpses, so it is possible that they desire living bodies only.
Restless spirits, until they possess a person or animal, do not present much of a threat on their own, relying on sheer numbers to overwhelm opponents with any defensive skill.
Abilities
Thought-reading: Spirits can become aware of a person's darkest thoughts and secrets, and have used this ability to torment both Farnese and Guts.
Possession: Accursed spirits can enter the body of an individual and control it, leading them to commit acts they otherwise wouldn't--usually heinous and violent. Collette was possessed by a spirit and soon after decapitated her father. Farnese was forced to strip naked and sexually assault Guts as well as mutilate her own genitals with the Dragonslayer. Accursed spirits can also enter the bodies of animals, using them as more durable ways of attacking victims. Doing so produces an effect very much similar to the Kushan pishaca. So far, dogs and horses have been shown as victims of possession by accursed spirits.
Mad Dogs
Mad dogs are stray canines that have been indwelled by accursed spirits. The dogs gain human-like faces with wide eyes and much longer, sharper teeth.
Mad Horses
Mad horses are horses that have been indwelled by accursed spirits. The horses gain wide, expressive eyes and a more human-like jaw, and can speak the human language. One mad horse in particular attempted to force itself on Farnese before being swiftly decapitated by Guts.
History
From the incident of the Eclipse onward, accursed spirits assault both Guts and Casca every single night, unless they enter an area under lingering protection of some sort, like that of the elves. From sunup until sundown, Guts is plagued by these beings and must fight them off until sunrise. For this reason, Guts typically stays away from other people for their own safety. The one time he relented and accepted help, it resulted in a father and daughter being brutally slain.
Accursed spirits play a larger role in the Conviction Arc. In a desperate bid to protect its mother from torturers, Casca's child creates an Interstice, summoning a horde of accursed spirits into the area. While this has the intended effect of repelling the torturers from Casca for the time being, it also has the disastrous effect of drawing in so many spirits from the combined presence of both Guts' and Casca's Brands that the Tower of Conviction begins to collapse around them. Not only that, but after this incident, Mozgus and his followers become convinced that Casca is a witch and intent on burning her alive.
Accursed spirits begin to have less presence after this arc, after which point Guts meets Schierke, who is capable of dampening the effects of his and Casca's Brands with a talisman.
Berserk and the Band of the Hawk
Accursed spirits are an enemy in the 2016 game Berserk and the Band of the Hawk.
These spirits make up a large bulk of the fodder enemies from the halfway point of the game onward, and the player may regularly be asked to kill up to a thousand of them in one sitting. They are no more dangerous than any other fodder enemies.
Accursed spirits are also present in the Endless Eclipse. Here, they serve a more challenging function as Mad Horses. On certain stages, Mad Horses may appear to pursue players across the map and violently and relentlessly attack. The player will be tasked with either defeating them or escaping from them, and the latter will be extremely difficult due to the fact that the Mad Horses will follow the player to the next layer even from as far away as half the map, and move extremely fast. Defeating them is also no easy task, as Mad Horses have immense health and durability, easily outpacing some bosses, and will restore 3% of their own health every few seconds. However, a large reward can be gained for putting them down.