The game is conducted from a third-person perspective. While the gameplay operates similar to Resident Evil – fighting monsters and solving puzzles to further explore a location – Eternal Darkness maintains notable differences in gameplay style, some of which distinguished it from other survival horror games of its time. An in-game map tracks a player's bearings. The inventory system stores weapons and items that can be used to solve puzzles. Some of which can be combined with other objects, even enchanted with magick, for different effects.
Combat focuses on a simple targeting system. Players may attack in a general direction, or lock-on to an enemy to focus on individual parts of its anatomy. Decapitating most enemies effectively blinds them. There are many classes of enemies the player must either defeat or avoid. Each class also comes in a few varieties, and subtle differences between each variety exist as well, having slightly different appearances and traits. Some of the more common enemies vary very little. Usually only changing in the hue of their skin, but the larger boss enemies tend to vary quite significantly.
The narrative of the game's story switches between two phases. The main phase focuses on a series of chapters in which players take control of a new character each time. The other phase acts as an intermission. The game boasts twelve playable characters, split between four distinct locations, and from different periods of time. Each of whom are different in terms of the game's three main parameters – health, sanity, and magick – and have access to a small selection of weapons that they can use in combat, though what they can use is determined by the time period, or era, that they lived in. For example, characters from the more ancient eras are restricted to mostly melee weapons such as swords, with the occasional crossbow or throwable. Meanwhile, characters from the modern era have more access to ranged weapons, including modern-day firearms.
The story features multiple paths that can be taken. This choice not only determines which of the game's three other antagonists are aligned to the plot, but it also has subtle effects on the gameplay in chapters and intermission periods. Some changes include slight differences in puzzles and items, but most changes revolve around enemy placement, which will determine how the player engages them. This can even have an effect on the relative difficulty of the game in certain situations. Red tinted enemies, for example, are tougher than their counterparts. Making the "red" story path a kind of unofficial hard mode. After the game is completed down one path, it becomes unavailable in future playthroughs, until the player completes all three paths.
The crafting menu allows the player to experiment and discover new spells
Magick can be used by most characters and consists of spells that can be used to damage opponents, protect characters and heal them, and be used to solve certain puzzles. The player is also able to assign spells to buttons for quick-use during the game. After discovering a spell it can be used in subsequent chapters and intermission periods. All spells are fundamentally affected by what alignment rune is used to power them. The game incorporates four types – Red, Green, Blue, and Purple. Each alignment affects spells on a specific parameter. On top of that, they operate on a rock-paper-scissors principle of gameplay. All spells require the player to combine a series of Runes together in order to cast them. Runes can be freely experimented with by the player. This robust mechanic of experimentation has been praised by game critics as unique, and something that sets the magick system in this apart from most magic systems of most other game titles.
Sanity effects will try to scare the player directly
The other distinctive gameplay aspect comes from "Sanity Effects", the game's standout concept that Nintendo patented.[3] Upon beginning the game's second chapter, players must keep watch on a Sanity meter – a green bar which decreases when the player is spotted by an enemy. As the bar becomes low, subtle changes to the environment and random unusual events begin to occur, which reflect the character's slackening grip on reality.[4]
While minor effects include a skewed camera angle, heads of statues following the character, and unsettling noises, stronger effects include bleeding on walls and ceilings, entering a room that is unrealistic before finding that the character never left the previous room, the character suddenly dying, and fourth wall breaking effects such as "To Be Continued" promotions for a "sequel", and simulated errors and anomalies of the TV or GameCube. While the latter does not affect gameplay, they can be misconstrued by the player as being actual technical malfunctions.