This article is meant to give you a brief overview of the flavour, strengths, and weaknesses of the seven factions in Call of Cthulhu. Let me stress that it is not all encompassing, and that a lot of the fun of CCGs is in coming up with weird and/or interesting combinations and strategies. With that said, let's get to it...
These guys are the cops who know a little more about the universe than is probably good for them. They're willing to accept that more is going on in this world than we've been taught to believe, and they're ready to blast the crap out of it.
Strengths: Wounding, wounding, wounding. These guys have almost all the wounding events and generally reign supreme in the combat struggle. This is also the faction that has the market cornered on resource destruction.
Weaknesses: Like all the human factions, they are vulnerable to insanity. The terror struggle comes first, and they'll often lose someone to that before they even get to shoot down the monsters. The cops also don't seem to read very much, because their a bit low on the Arcane icons.
Balance: This faction is pretty well balanced between characters and events, without one really outshining the other. Decent amounts of toughness and fast, as well as great wounding events all around give this faction a pretty solid feel. A couple always-want cards include Undercover Security (EE C4), Hired Muscle (AE C11), and Shotgun Blast (AE C22).
Partners: Some good factions to mix with this include Miskatonic (if you want speed), Syndicate (if you want sneakiness), and Cthulhu (for even more destruction).
The professors who are a little too interested in mystery than is good for them. These guys would walk into a cave unarmed to discover the source of that interesting and unknown chanting.
Strengths: Investigation and Arcane. No other faction has the last two struggles as wrapped up as these guys. Unfortunately for them, there's the first two that they tend to have less focus on. They're also the masters at card drawing.
Weaknesses: Just about everything else. They're quick to go nuts, and easily wounded.
Balance: They don't have the toughest characters out there, only some cheap and quick ones. This faction generally leans more toward the great events that they have, most of which are all about helping your characters survive, despite sucking. Stuff to keep you going include Binding (AE C44), Crafting the Elder Sign (AE U46), and Local Historian (UT U23).
Partners: They work well with Agency (who provide the muscle), Syndicate (who help them avoid fighting), and Yog (whose recursion abilities nicely compliment the card drawing and filtering that MU has).
The seedy underbelly of the city. These guys know what's going on, and are trying to figure out how to turn it to their advantage. Anyone care to take bets on whether or not Cthulhu will reawaken and devour us all? I'm giving 5:1.
Strengths: There's a lot of avoidance and sneakiness to be had in this faction. Events that cause your opponent's characters to exhaust, ones that switch stories, ones that lower skill when they don't suspect it. There's no particular icon which they excel at, but rather a bunch of cards that give you a little of everything.
Weaknesses: They are second only to Hastur in horrible characters. Apparently, all the good characters must have switched sides and join up with the cops, cause there's none to be found in this faction.
Balance: It's all about the events in this faction, because it sure ain't the characters. Most Syndicate decks will be pretty event heavy, using those cards to make up for the fights that they just won't be winning. Exposed (AE U77), Intimidate (AE C80), and On the Grift (EE U80) are just the tip of the iceberg for these guys.
Partners: Mix in these guys with a dose of Agency (for character removal), Miskatonic (for extra tokens while avoiding the scary guys), or Cthulhu (for some decent characters to go with those interesting exhausting events).
The namesake of the game himself, along with his various minions and cultists. They tend to be mostly concerned with killing or destroying everyone and everything. If they themselves get knocked off in the process, well it was all in a day's work.
Strengths: This faction has some really solid characters with a good mix of terror and combat, and the deep ones in the faction also have arcane to round things out nicely. This faction has mean support cards and some really solid, and often "big" events. A minor strategy you'll find here is blanking out textboxes and destroying Attachments, but what they really thrive on is destruction and sacrificing.
Weaknesses: Well there isn't a lot that the big green guy can't do. However, his big problem is that he doesn't play fast. All the characters tend to be expensive, and the destruction events are just as costly because they tend to be very big effects. That said, once he gets rolling, he's tough to shut down.
Balance: Overall a good blend of cards, but I'd have to say that the true strength of this faction lies in its great characters and handful of good events. You'll generally be playing with the same cards in almost every Cthulhu deck you make. Try out Sacrificial Offerings (UT U66), Great Old One Rises (AE U110), and Keeper of the Golden Path (AE U96) for just a taste.
Partners: These guys go well with just about anything, but here's a couple for you anyway. Try out Agency (for mass wounding/destruction), Hastur (for lots of insanity fun), or Yog (for a ton of arcane and icon supplements).
"He who must not be named except he gets named a whole lot" is all about the crazy people. Whether he's sending them to the institution, making sure they stay, or leaving someone in a debutante's ballroom never to return, this boy's out to have fun.
Strengths: Control, control, control. This faction has the book written on cancellation, as well as hand control. Insanity also plays a major part in this faction, but it generally needs to be paired with another faction in order to get the terror icons you would usually want. This is also the faction with the most powerful deck discarding abilities.
Weaknesses: The characters. These guys must have written the book on how to suck at recruiting, cause they have no one. There are like three good characters and you'll use them in just about every Hastur deck. Byakhee Servant is one of them: can you name the others?
Balance: This faction is all about its events, and with good reason because these are very powerful events. Often, you'll pair this with your main faction just to get some of the great control events Hastur has to offer. Byakhee Attack (AE U123), Power Drain (AE U139), and Xanthophobia (AE C142) are some solid utility cards.
Partners: This discard theme is pretty much a mono-faction base, but other strategies to work with include Agency (for even more control in the form of character destruction), Cthulhu (for the insanity strategy), and Yog (for deck manipulation and recursion).
The followers of the Key and the Gate do a pretty good job of organization. They have a lot of cultists following them around, and they can pretty much find or bring back anything that you're interested in.
Strengths: They definitely have the market corned on recursion, whether it's characters or pretty much anything. You want something back from the discard? Talk to Yog. They've also got some great filtering cards to help you find what you're looking for quicker. This is also the only other faction with any discarding strategies available to it.
Weaknesses: Surprisingly enough, a good percentage of their characters don't handle terror too well, so insanity can be a problem for these guys. Their characters in general are okay, but usually nothing special. You can many times find better ones with similar or better stats elsewhere.
Balance: These guys are definitely event-based, but at least their characters don't suck like Hastur. These guys tend to be pretty complicated in what they do well. Most people will tend to look at them and think they're just okay, but once you realize the power of the filtering and recursion that these guys have mastered, you'll see some new roads opening up for you. Cards to mention include Unspeakable Resurrection (AE C172), Journey to the Other Side (FR C101), and Son of Yeb (EE C161).
Partners: Fun stuff to try includes Miskatonic (for more card filtering and drawing ability), Cthulhu (for general monster fun), and Shub (for more character recursion and cultist hilarity).
The Mother and her endless spawn have one basic goal: more spawn. If there's a machine in this game, it's this faction's ability to crank out monsters. Rabbits can't even come close to what these guys can do.
Strengths: Characters. They have great characters, often with toughness, and real good mixes of terror and combat. In fact, many of the characters have both terror and combat, a definite rarity in every other faction. This faction also reigns supreme in resource acceleration, and support card destruction.
Weaknesses: They're fairly well rounded, but if there's anything they're not good at, it's character removal. They have precious few ways of getting rid of characters, so you'll often run into stalemates where the characters just build up on either side and you can't get rid of them.
Balance: This faction is definitely character driven, but does have a few key events that help it make a big difference. They also do pretty well with character recursion, which is to be expected from the All-Mother. Some key cards for any good Shub deck include Mi-Go Worker (AE C191), Hungry Dark Young (AE C187), and Burrowing Beneath (AE C197).
Partners: This faction tends to be the dominant one if you play it, and works well on it's own but I would add Agency (for the character removal), Miskatonic (for investigation and more speed), or Yog (for even more recursion fun).
I hope you enjoyed this romp through the factions of Call of Cthulhu. Obviously, this was just the tip of the iceberg, but I hope it gave you a good handle on what each group is like. Try out the suggested faction combinations, and then see how you might fit together factions that I didn't mention. Look at the weaknesses and strengths mentioned above and see what combinations you can come up with where one faction's strengths help out the other's weaknesses.
Also, you may have noticed that I don't really mention support cards. That's because support cards are inherently less powerful. They are exactly what their title suggests; they support the rest of your strategy. And with very few exceptions, they don't win you games. Because of that, and because the designers know what they're doing, there is no particular faction whose strength is in their support cards. Having said that, there are some key support cards just like there are key characters and events. Deciding what support cards are critical to what strategies is left as an exercise for the reader.
Lastly, the cards I suggested are all good cards, but obviously there are lots more within the strategies and strengths that I talked about. I simply tried to stay with common or uncommon cards, because it's easier to get started it off that way. Also, they are largely from Arkham Edition, because that's where a lot of the strategies of those factions were defined and so there are a number of cards set out for that purpose.
As usual though, this is not the complete picture and I invite you to dive into this game and discover the more intricate strategies to be found within each faction.