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The defining feature of Grave Seasons is that one of the townsfolk of Ashenridge is a supernatural serial killer. The farming sim loop sits on top of an ongoing murder mystery, and the player gradually gets wrapped up in the killings as they settle into town.
The studio frames the mystery as fundamental to the game rather than bolted on. Every relationship, schedule, and daily activity can feed back into the investigation, so the cozy routine and the detective work are meant to be the same loop rather than separate modes.

A Randomized Killer
The killer's identity is randomly selected from a subset of the named cast at the start of each playthrough, and the same player will never see the same killer twice in a row. Because the murderer changes between saves, the clues shift as well, so no two playthroughs follow the same path to the truth. This is the core of the game's replayability: the same town and cast can hide a different killer each time. The killers themselves are described as supernatural creatures rather than ordinary criminals, which lines up with the monsters seen in early footage of the game.
Murders Across The Year
Murders unfold over the course of the in-game year. As the seasons change, the killer continues to strike, putting the player's new peaceful life and the townsfolk at risk. With four seasons in a year, that adds up to at least four potential victims per run. Each month gives the player a limited window to spot who is in danger and craft a countermeasure to head off the killing, if they choose to intervene at all. Mayor Fitch has been confirmed as one of the possible victims rather than a possible killer; other potential victims have been hinted at in the release-date trailer but not enumerated.
A public hands-on demo gave a concrete look at this. The player walks into the mayor's office to find him dead, with clues strewn about the scene to collect, which fits Mayor Fitch being one of the confirmed potential victims. See Investigation and Clues for how clue collection works.
The player's job is to investigate the residents, gather clues, and work out who is responsible before more people die. See Seasons and Calendar for the structure of the year and Investigation and Clues for how the player gathers evidence.
Monsters In The Night
Although the killer wears a townsfolk's face by day, the kills involve creatures that hunt in the woods around Ashenridge. One of the most prominently shown is a wolf-like beast with glowing red eyes, which drags Pilar through the woods on a trail of blood in the official gameplay trailer. Early coverage suggests there are several distinct supernatural threats in the game. See Monsters for what is currently known about these creatures.
Influencing The Killer
The player is not only a passive observer of the killings. Choices made during a playthrough, including who is allowed to live or die, reshape the evidence and the story. If the player identifies the next victim in advance, that person can be protected, which branches the narrative in a completely different direction. It is also possible to influence the killer to stop, or to push them further over the edge, and the killer's mental state at the close of the year affects which ending the player reaches.
Romancing The Killer
Because the killer is one of the townsfolk and relationships are a core system, the player can end up romancing the killer. Whether this happens by chance or on purpose, it is an intended part of the experience. See Romance and Relationships.