Despite working with relatively primitive visuals, this game is a true gem and worth anyone's time.
Each chapter follows the adventures of one of a dozen or so key historical figures who are sucked into a world of madness when they find that there are forces outside the realms of human knowledge plotting against mankind. You play each of these characters in turn and each has their own specific advantages and disadvantages. In between chapters, you take control of Alex who is also the last of this line of unwitting mortals to get drawn into the plot of the game's antagonist, Pious Augustus. Augustus is an immortal servant of one of three Ancients, powerful godlike beings working toward their ascension which will bring about the end of humanity.
The game owes much to H.P. Lovecraft's so-called Mythos which dealt often with similar themes and in similar ways. It wouldn't be so difficult to substitute Cthulu for Xel'lotath, for example. If you like Lovecraft or Lovecrafitian stories, this game will instantly appeal to you with it's stand-ins for the Necronomicon, Ancient Ones and it's focus on a history-spanning descent into madness.
And yes, sanity does play a huge role in the game as well.
The basic gameplay involves taking control of one of the characters and exploring one of four primary locations. Since there are more characters than locations, you'll revisit each a couple of times at least. You start as Alex in the Roivas family mansion and once you discover the Tome of Eternal Darkness, you begin to read the stories, chapter by chapter, taking on the roles of the people described. The other locations include a mysterious underground lair in Persia, a temple in Cambodia, a cathedral in France and the Roivas mansion in Rhode Island. Revisiting each place may seem dull, but the designers do an excellent job marking the passage of time so you don't feel like you're doing the same things over and over. In fact, for the most part the minor puzzles you must solve to get where you need to go don't have the same solutions from one time period to the next due to things being changed as time goes on.
In each character's story you'll get a variety of interesting weapons with which you'll battle an assortment of creatures from undead skeletons and mummies to less easily classified creatures which arrive from other realms or dimensions according to Augustus' designs. Combat is designed around a simple mechanic of pulling the left trigger to target an enemy and then, moving the thumbstick, pinpointing the target to either torso, head or limbs (where applicable). Most of the weapons in the game are melee weapons so you'll spend a lot of time hacking off arms and heads with swords, axes and clubs. A few of the more modern characters can get guns and there are some other ranged weapons available as well but most of the combat takes place in close quarters so except for making it easier to avoid attacks while dealing damage, they don't help all that much.
You also learn through the course of the game a number of magic spells. The magic system in the game revolves around combining certain runes with power directed from one of the three Ancients for different results. Each Ancient has an associated color and each one trumps one of the others in rock-paper-scissors style. The runes are illegible at first so you must also collect a codex to translate them, then you need a scroll which describes the spell and indicates the rune components required. The system works well and even allows for some experimentation (you can figure out certain spells long before you get the associated scroll). When actually casting the spells you must wait for each rune to be read in turn without moving or attacking lest the spell fizzle. Later in the game when you get spells that require five or even seven runes to execute, it becomes important to your survival that you don't waste your magic power on fizzled spells that got interrupted because you cast them without having enough time to complete them.
In addition to combat and magic, you also have to concern yourself with your sanity. Each time your character encounters more of the truth about the machinations of the Ancients, they begin to feel their mind unravel. Seeing the walking dead, for example, can cause you to go a little crazy. Sanity is measured like life and magic with a meter. When your sanity is completely gone, sanity-decreasing events start to inflict physical damage... but that isn't the most significant aspect of sanity loss.
What really happens when you lose sanity is that the game starts to mess with your--the player's--head. At first it might be something simple like a knocking on a door you can't locate. Or you could begin to hear sounds of a baby crying. As your sanity decreases even more you end up hearing bloodcurdling screams, and having visual hallucinations such as blood running down the walls. If it gets bad enough the game will even start to affect you beyond the game itself, such as having you walk into a room and find yourself on the ceiling. Or you could see what looks like a blue screen of death. These effects are thrilling and highly effective and with at least a couple dozen different effects you're likely to be surprised by at least a few of them during the course of the game.
The puzzles in the game are generally pretty easy although they are fairly good about avoiding contrivances like block-pushing and there is enough variety to them that they don't get in the way of the game. A lot of the puzzles from the chapter levels relate directly to something in the mansion as well so when Alex finishes a chapter she usually has new knowledge that helps her find the next chapter page. It is in fact this chapter page hunt that is probably the most contrived aspect of the game, but it works well enough to keep the story moving.
When all is said and done the game itself is enjoyable but not remarkable; plenty of games have offered action adventure of this type before. Eternal Darkness executes it competently but only really shines because of how well all of the action fits in with the sanity system and the plot of the game. And in the end it is really the story that shines the brightest. Fortunately you feel like you're right in the thick of the story the whole time so it's not like the game goes on for long periods without any interaction. Generally speaking the cutscenes are short and nearly always tense and interesting so you won't feel like you're bored and wishing you could get back to playing.
That Eternal Darkness manages to be a truly great game is somewhat remarkable considering the labored development process and the fact that it started life as a Nintendo 64 game. Where this is most apparent is in the graphical presentation which is pretty rough. The animations are nicely captured and the graphics are passable enough to get done what they need to get done, but there are plenty of places where you'll find yourself wishing they looked a little--or perhaps a lot--better. Most of the game's horror and gore takes some effort on the player's part to be effective because it simply doesn't look convincing enough when stacked against the Resident Evils, Silent Hills and others which are able to work on a more visceral level due to their high quality visuals. It's really the game's only glaring flaw but aside from a handful of moody lighting effects it's pervasive and that makes it a fairly serious flaw.
Perhaps as way of compensating for this, the rest of the presentation is rock solid. The audio in particular is phenomenal, with top notch voice acting throughout (including during some pretty tense and emotional scenes). Only a handful of the pervasive voice performances are anything less than stellar and the sound effects are pretty much spot on throughout. The game's menus and HUD are logically laid out and designed with plenty of appropriately macabre touches. And while the in-game graphics are kind of sad, at least the pre-rendered cutscenes look good.
All in all this is a fantastic game, and whether you are a Lovecraft Mythos nut or never heard of him but like Resident Evil style games or perhaps if you normally shy away from straightforward horror games but are looking for an exciting adventure/action title, you really can't go wrong with Eternal Darkness. The game will take about 10-15 hours to beat depending on whether you ever get stuck or not but the game can be played through three different times (once for each Ancient) which will give you three distinct yet similar sets of cutscenes and a special bonus ending when you beat all three. With maybe 35 hours of gameplay it may not seem like much of a "purchase" title but it should be pretty easy to find either used or deeply discounted so anything under $20 I would consider to be a sound investment.