The Necronomicon is a fairly simple card game based, as you might expect, on the works of H.P. Lovecraft. It does an excellent job of making a sufficiently spooky game -- so good that you can even overlook the occasionally annoying mechanics.
The basics of the game are quite straightforward. You have a hand of five cards, and on your turn you play (or, rarely, discard) one and then draw one. Each player has a given amount of life (the starting amount of life is 40 at the beginning, but rises gradually over the course of the game), and the object is to reduce your opponent's to zero before your own does the same. Cards fulfill the general range of functions -- attack your enemy, defend or heal yourself, and so forth. Creatures do exist, but they don't behave like they do in, say, Magic -- if you summon a creature (and you may only have one in play; summoning another will simply replace your existing creature), it won't attack on its own; rather, it will counterattack whenever your enemy does damage to you. (A creature also can absorb damage from your opponent's creature, when it is activated.) This means that creatures are really not that useful overall. There are four other attributes in play. Two are Elder Defense and Arcane Power, which reduce the amount of damage taken and increase the amont of damage done by your spells, respectively. There are a fair number of cards which increase these, but there are also lots of cards which let you zap your opponent's power or defense, so don't get too attached to them. The third is Taint, which acts as poison -- it does its damage every round. Taint can be useful in the early games when characters don't start out with much life, but in the later rounds it doesn't really have enough time to do too much damage. (It also can never finish off a player -- it disappears when a player is reduced to 1 life.) The final, and most intriguing, element is your sanity. Most cards cost sanity to deploy -- in general, the more powerful and arcane the forces you're summoning, the greater the sanity cost. A few sanity-restoring cards exist, and if you discard a card, you recover an amount of sanity equal to its cost. Still, you will generally find your sanity gradually slipping away over the course of the battle. Should your sanity go to zero, you go insane. Going insane brings you one of a variety of delightful effects; these effects will generally hinder your ability to win your struggle, though they certainly do not prevent it. The pool of cards is not particularly large -- there's 36 different cards (not all of which are present at the beginning); it's not clear whether your deck contains these in differing numbers, or if there's just a certain random chance every time, though at least anecdotally some cards seem to come up more frequently than others.
While there is some strategy involved (as evidenced by the fact that the AI isn't very good at it), the game often does boil down to drawing the right card at the right time, which can be frustrating. There are two modes of play: in the main mode, you must defeat 30 enemies of gradually increasing power. Fortunately, as you defeat the enemies, your own power also increases depending on how well you do. As a result, it's often a better strategy to lose rather than eke out a close victory, since the latter will give you very few points and can leave you at a disadvantage in your next battle, while trying again to get a more overwhelming win can give you a chance to go up a rank and maybe even two, giving you an advantage. The challenge mode features 21 different challenges where the rules of the game have been altered, sometimes subtly, often grossly, and always in favor of your opponent. The challenges range from entertainingly challenging to extremely frustrating, and here especially you can see just how much the game depends on random chance. To illustrate how tooth-grinding this can be, in one challenge, you have a stipulation that you lose if you go insane. However, the opponent has in his deck a card which makes you instantly go insane. So, if this card comes up, regardless of how well or poorly you might have been doing, it's an instant loss! This gets pretty tiring pretty quickly.
Atmospherically, though, the game is fantastic. The graphics are excellent, the background music lends just the right air of eeriness to the proceedings, the sound effects, although very understated, also fit in perfectly with the game; even the typography has just the right feel. Alas, the game does have to make a few compromises -- since it can't show all five of the cards in your hand in their lovely detail, for instance, you instead have to click on a card to magnify it and then click on the Necronomicon to actually play it, which gets a little clunky once you've reached the point where you do know what every card does.
Anyway, overall this is a well-crafted game, but unfortunately it's a little bit too long -- you will undoubtedly find yourself frustrated by the randomness more than once during the course of the game (and even more should you attempt all of the challenges, though at least you can get an impossible badge for it). Still, the game environment is good enough that you can still enjoy it, if you don't mind a little bit of creepiness.