See also: Counter
A combo can refer to two things:
- Two or more cards with a particular synergy
- A cohesive strategy that relies only on two (sometimes three) cards
The first can act as a supplement or centerpiece to a larger strategy, and is typically the broader category. But when talking about a combo deck, it is the second that is being referred to.
In some combos, one card will make up for the other's weakness. In others, the two cards will play off each others' strengths. While combos of 3 or more cards are possible, unless one of these is a base card in every game (or at least one of the cards needed is simply a card class, e.g. village or +Buy/gainer) rather than a specific card, it's unlikely that you'll be able to find the combo in many games. Such combos really fall into the category of strategies rather than merely simple combos. As it is, depending on how many expansions you use, the odds can be as low as 1 in 616 that two specific cards will appear in a given kingdom. However, given that many players play 100's, if not 1000's of games, these odds are not too bad.
Original article by WanderingWinder.
 What characterizes the Combo?
A combo deck is a one that revolves entirely around a particular specific combo of 2+ different cards, generally getting 5-20 copies of the required cards in total. Once the combo is in place, if this has happened quickly enough, the deck should basically just win. This archetype does not deal with cards that work well together – i.e. it’s not just decks that have combos in them, a la Horse Traders-Duke, which is a Slog, or Warehouse-Treasure Map, which works together well but isn’t an entire deck, but rather deals with combos that are self-contained, the-entire-deck-and-gameplan-is-this strategies. Typically, these combos are fairly resistant to adding other cards in with them.
Usually, with more than two distinct cards, you are really talking about an engine, which is a bunch of good cards that work well together and draw a lot, but this is not a hard-and-fast rule. Even things like Hunting Party stacks and Minion decks aren’t combos – they’re engines – that’s just (strong) cards being used to cycle you through, which can sometimes be extremely powerful with few cards, but isn’t a combo – there needs to also be some particular synergy, a sum-is-greater-than-the-parts, but particularly in such a way that goes beyond the normal functionality of a card.
For instance, something like Worker's Village and Rabble pair really nice together, as they give you Actions, Attack, Buys, and draw – everything you need for an engine except money – but this is clearly not a combo, it is an engine, and part of how you can tell is that the parts are really modular – you might lose a little bit of efficiency by switching to another kind of Village or Smithy variant, but generally those roles can be filled by a number of other cards.
Something like Native Village/Bridge, on the other hand is very much a combo – it plays quite differently than other decks featuring these cards, and more important, you can’t get the same kind of functionality out of any other cards.
Some examples of Combo Decks include (not an exhaustive list!):
 The Native Village/Bridge deck
Buy nothing but Native Villages and Bridges. Use Native Village at every opportunity to put every card on the Native Village mat. When the time is right, empty your giant Native Village mat and mega-turn out with all your Bridges.
Buy 4 Stashes and multiple Chancellors. Every turn, play your Chancellor, put 4 Stashes on top of your deck, and get a Province. Repeat as soon as you draw another Chancellor. Scavenger makes this even more powerful, since it allows you to guarantee putting a Scavenger in your hand every turn. Plus you only need 3 Stashes.
 The Golden deck
Your deck is nothing but Copper, Golems, any amount of victory cards and other treasures, and exactly one Counting House. Every Golem puts your whole deck in the discard and plays the Counting House, which draws all your Copper.
 The Apothecary/Native Village deck
Have 8 Copper, 1 Native Village, and tons of Apothecaries. Never use Native Village blindly; only use it to filter out green cards that your Apothecaries leave on top of the deck. Once you start humming and your Apothecaries draw or , your sole Native Village sucks up the new green card every turn, while your Apothecary leaves an Apothecary on top of the deck, essentially guaranteeing that it can keep going indefinitely.
 King's Court/King's Court/Scheme/Scheme
Adding just about anything else guaranteesor every turn thereafter.
Tactician gives you 10 cards. Vault draws you 2, to a total of 11, and you can discard up to 9 of them for . The remaining two cards are a Tactician and something you discard to the Tactician to repeat this process.
Tactician gives you 10 cards. Black Market plays all your Treasures. Another Tactician discards your last card and gets you back to 10 cards next turn. Black Market saves all your Treasures from being discarded, so it’s basically like playing Dominion with 10 cards every turn.
 What’s Good For Combos?
Again, this is all dependent on what combo you’re playing, but there are some general tips. Usually, there is some weakness of the combo.
This is most commonly an attack – e.g. the Golden Deck is vulnerable to junkers and discarders. Chancellor/Stash or Scavenger/Stash is very vulnerable to discard. Scheme combos are vulnerable to Minion, and if it’s a large enough Scheme chain, Fortune Teller.
There are also other kinds of holes – for instance, combos which eventually attack the opponent into submission can be vulnerable to reactions. Native Village-based combos are extremely vulnerable to Possession. So watch out for these things, know when they counter you, if they can be fast enough, etc. And know how to use them to counter combo decks if you are on the other side.
Combo decks are also in general vulnerable to not getting up and running in enough time. What ‘enough time’ is depends on the combo. Others need enough expensiveon the board, or at least 50% in many cases, to be available.
The biggest threat to making this happen is generally a strong, fast engine, as they can certainly outrace Combos, most normally if they have strong trashing to kick-start them. Rush decks also pose a serious threat in being able to finish things off too quickly for the combo to get in place.
Mirrors become strange things. Either it is a race to get the combo up first, which is a combination of 1st turn and luck, generally, with some skill on the order of the build, or very often it will end up in a 3-pile ending, where you want to build as long as you can so that you can cash out, but if you DO cash out too early, then this gives them a lot of time to build up for a bigger turn, and if you build too long, you run the risk of them three pile ending you. So the timing of when you pull the trigger, particularly if your combo is one that can go off as a matter of scale, is a huge skill in this kind of matchup. If the three pile ending is very likely, you need to watch out for defensive greening. Sometimes, a seemingly random Duchy can be VERY good – it gives you the lead, will be good in the long run, and most importantly, it can stop your opponent from being able to buy more engine components, by utilizing the threat of that three pile ending.