Combo: Horn of Plenty and Mandarin
|Horn of Plenty / Mandarin|
|Set(s)||Cornucopia / Hinterlands|
Horn of Plenty and Mandarin is a combo that, once set up, guarantees a Province or Colony every turn. With enough differently-named Treasures in play, Horn of Plenty gains a Mandarin, whose on-gain ability top-decks all your Treasures to do the same thing again next turn, leaving enough to buy a Province or Colony.
 How it works
The Colony version is easier to set up, as it needs no other cards than those two in the supply. The goal is to set up a hand of exactly one copy of each basic Treasure (Copper, Silver, Gold, Platinum) and Horn of Plenty. You play the 4 basic Treasures first, and then the Horn of Plenty to gain a Mandarin. This causes you to put back all 5 cards on top of your deck. Then you have exactly to buy a Colony. Rinse and repeat on the following turns.
For the Province version, you need an extra treasure card that gives you at least each time you play it. This is usually a Silver or Gold variant card like Stash, Royal Seal, Cache or Harem. Bank, Philosopher's Stone or even Diadem have a high opportunity cost, but it works with these too. It could also work with Ill-Gotten Gains, but it's hard to setup when you gain a lot of Curses. Venture also works as long as it doesn't find another Horn of Plenty or even a Potion. Contraband usually doesn't work because your opponent wouldn't allow you to buy a Province.
 Setting it up
This is really hard to set up if there are no supporting cards, and this can heavily depend on shuffle luck. Cheap cards like Haven or Herbalist can help you to put back or save the Platinum or Gold for the next turn. Heavy trashers or cards that increase hand size - especially Tactician - are good enablers too. Another possibility is to wait until at least one copy of each key treasure is in your discard pile, and then buy a Herald for , overpaying by , and moving the 5 treasures you need from your discard pile to the top of your deck. This may require a clever way to get to at the right time, such as coin tokens.
This combo doesn't work with any discarding attacks because you need the 5-card hand each turn. Also, this combo only works as long as there are still Mandarins in the supply. You can break it by gaining Mandarins.
A similar combo involves using Counterfeit as a source of +Buy instead of Horn of Plenty as a gainer, but works on the same principle. A hand of four Golds and a Counterfeit can produce and two buys (don't trash any of the Golds with the Counterfeit); then you can buy a Province and a Mandarin, top-decking the five Treasures.
As of Empires Capital can be substituted for a super-powered version of this combo, only 2 Capitals and a Copper are needed. The top-decking from Mandarin neatly prevents the Capitals from being 'discarded from play' and therefore avoids the debt gain trigger. 3 Capitals allows you to pull the same trick on Colonies.