Two-player card games occupy an interesting niche in the world of modern boardgaming. They're typically either "person and SO" games that you'd play with a partner as something to do other than watch TV or they're hardcore competitive, tournament-style games like Magic: The Gathering. The former tend to be fairly straightforward, with relatively simple systems that both players can easily remember. The latter tend to have rules the size of phone books and often require being heavily involved in a competitive scene to really display their depth. Reiner Knizia, the near-legendary designer of games like Tigris and Euphrates, Through the Desert, and Modern Art, designed a two-player card game that aspired to the qualities of both. That was Blue Moon.
Blue Moon is a very simple game with an odd theme that kind of reflects the typical criticism of Knizia's designs, in that the game's mechanics don't really reflect the trappings laid upon it. T&E doesn't really have much to do with ancient Babylon, but it's still a remarkable game. Blue Moon is based in some other dimension where there is competition between two heirs to the throne of Blue Moon City after a special dragon has been slain. The two heirs are trying to rally the different peoples of this dimension to their side to determine who will take the throne. The way to do that is to gain the favor of the other dragons, which you do by winning fights with those peoples against the allied peoples of your opponent. The fights are won with simple numbers in two elements: fire and earth. If I have a higher number in the element of a particular fight than you do, I win the fight and attract one of the dragons. The game ends when we both run out of cards. Whoever has the most dragons wins.
So, it's not a high degree of complexity at first glance. But like most Knizia designs, the magic is in the depth; not just in experienced play like knowing when to use boosters or supports to win certain fights or when to play certain characters at the best time, but also in basic strategy, like when to back out of a fight that there's no sense in continuing, rather than giving your opponent an even bigger win. There was a fair amount of gameplay in the two peoples that came with the main set, the Vulca and the Hoax. But it was clear that the other seven peoples (Aqua, Flit, Khind, Mimix, Pillar, Terrah, and last-to-be-released Buka) were intended to bring the game to its full vision, since some of those people were included as one-offs in the main box (and are also scattered across the others, except the Buka) and all of them are assigned a score in Moons that are intended as a framework for deckbuilding, wherein you can mix cards from several different peoples in order to design a unique deck. The game was popular enough in Germany that an organized league existed for a couple years, with people bringing their own deck designs to tournaments, just like MTG.
Each of the peoples has their own general identity, as well, that often breaks the rules of the game in Cosmic Encounter-style fashion. The Khind, for example, break the rule of having to replace characters played each turn by allowing you to pile members of the same gang on top of each other, to create a larger and larger threat. Similarly, the Mimix allow you to play two characters at once, if they share a name.
Vulca: As you might expect, the Vulca are heavily slanted toward Fire, with the highest Earth value being 4 and Fire values reaching as high as the 7 of Flamebreath the Dazzling. Staying in theme, many of the non-character cards (booster, support, leadership) are focused around offense and/or removing your cards in play so that your ability to restrain that offense is lessened. Fitting that kind of torrential approach, they also have several card draw options, which can keep your hand ready, but also means that you have to be aware that you'll run out of cards before your opponent, who then may be able to even the dragon score by winning fights you can't participate in.
Hoax: The other main box people are more evenly-distributed, element-wise, without a value of more than 5 in either Earth or Fire. But the Khind are also a good introduction to the rest of the peoples because they take advantage of many of the other mechanics in play, such as Flit's Retrieve and Pillar's discard tendencies. The Hoax also rely much more on support cards, such as the 3/3 Tome of Wisdom, trying to build an element game off of multiple cards in play, rather than the one-shot approach of the Vulca.
Aqua: One of the more complicated peoples, they tend to rely on combos of cards, using Free characters to cycle through their deck and then refill it while trying to get generally weaker character cards in play that are Protected (i.e. can't be targeted), alongside effect-based supports like Irresisitbile Sirens that encourage the opponent to stop competing in fights that they could otherwise win, or Water of Conjoining that can turn one of their weaker cards into an effective challenge.
Flit: The birdmen are based around two main mechanics: Retrieve and Pair; the former for characters, the latter for boosters. Being able to play and replay characters like Highchirp against the Khind means that they can present a winning condition to almost any fight, even with their low value characters (nothing above 4 in the whole deck.) They also only have one support in their deck, which means that if they're going to win fights, they're going to do it from their hand... which also means that you have to be wise about how often you use Retrieve, since it will limit the number of new cards you'll be drawing each turn.
Khind: The green-haired gangers are one of the more difficult peoples to play with, since they're extremely draw dependent. If you draw two or more of the same gang, you're rolling. If not, you have a bunch of small fry that often won't effectively contest fights. You can effectively halve your opponent's values with the right members of the Cool gang and the Top gang also targets your opponent's active cards, so you can often stay in fights longer than you expected; depending on your support and leadership draw effects to get you to the right combinations, but it's challenging.
Mimix: Similarly, the Mimix are rather draw dependent. Although they have one of the highest average value ratings for characters in the game, most of them lack special power text, which means you're hoping to overwhelm your opponent with raw strength, and that often requires the proper cards in hand to take advantage of the Pair mechanic, which is the central identity of Mimix. Timely play of their Shaman characters, which allow you to search out specific cards in your deck or discard pile, is often key to victory. Like the Khind and the Vulca, the Mimix are extremely character-oriented, which will often determine your draw strategy.
Pillar: The bug people are one of the more thematic decks, based on their Butterfly supports, their Potion boosters, and the dread Caterpillar boosters, which are all Free and designed to wreck your opponent's hand via discard requirements. Somewhat like the Aqua, it's one of the more trick-heavy decks, without great element power among its characters, but with enough fancy tricks to make up for that lack of raw power. It's one of the decks that actively forces your opponent into choices, not just about this turn, but future turns. The Butterfly cards that make your opponent reveal their hand can also mean demonstrating just when playing a Caterpillar is useful.
Terrah: This deck is essentially the counterpart to the Vulca for Earth, with values in Fire not rising above 4 but with Earth values going as high as the 7 of Megalit Mountainshaker. In contrast to the Vulca, who spend a lot of their tricks on removing active cards, the Terrah spend more time on hand manipulation and destruction, trading the speed/immediacy of Fire for the slow, steady drumbeat of Earth as your opponent's hand becomes less useful the longer you stay in the fight. Like the Vulca, however, the main strategy is to overwhelm the opponent with raw power.
Buka: The pirate people introduced the concept of bluffing to the game, where cards are played as placeholders with 2 power and your opponent has to risk guessing whether you're lying about their element. Victory in the current fight is dependent upon them choosing properly or not. The Buka also expanded upon the mechanic of Influence cards that another expansion introduced, by playing Ships that could carry characters to be released whenever the Buka player decided that a fight had to be won. It's a deck that requires a decent knowlege of competitive psychology: knowing when to bluff, seeing how your opponent is reacting to the loaded ship next to the board, etc.
One thing external to play that's worth mentioning is that Blue Moon was one of the more beautiful games to emerge around that time, with an art style kept in the enchanting/mystical genre and which made the slightly oversized cards worthwhile in order to really enjoy the imagery. The two Emissaries and Inquisitors decks both added cards to the regular game play and opened up the possibility of deck design with the various Inquisitors that set different advantages, as well as adding to Moon limits for cards that could be incorporated outside your base people. Many people saw these two decks as adding unnecessary complication to what is normally viewed as a fairly sleek and straightforward design and it's difficlt to argue with them. Influence cards and the Hyla and so forth are interesting, but there was enough gameplay within the peoples themselves to keep any pair of players occupied for a long time. Other than the enabling of deck design, I often end up wondering if the rest of the cards introduced by the two E&I decks were design elements that Knizia had really fleshed out.
Is that why it's one of his games that never really reached the elevated status of most of the rest? Was there too much going on for a "simple", two-player card game? Or was there not enough to the gameplay that is essentially a form of War (whoever has the higher-numbered cards wins)? Regardless, I still think Blue Moon is one of Knizia's more interesting designs and an ideal two-player game for any pair of players. I'm eager to get it off the shelf at every opportunity.