Darkrai/Hydreigon consists of, obviously, Darkrai and Hydreigon, and almost always Sableye. But similar to Klinklang of last format, a whole variety of techs can be added, such as Sigilyph, Shaymin EX, Mewtwo EX, just to name a few. Darkrai is a good all-around attacker, with an amazing attack and a very helpful supporter. Sableye is super useful in the first few turns, and it can even be used later in the game to get back crucial trainers. Hydreigon is basically the core of the deck, because it gives the deck it's versatility and is also what allows it to use Max Potion so well.
Main StrategyThe main strategy of Darkrai/Hydreigon is to use Darkrai to deal a very nice amount of damage, and using techs to deal with anything your opponent might send out. With Hydreigon, you can easily stop attacking with one pokemon and start immediately attacking with another. Hydreigon also allows you to abuse Max Potion, which is immensely helpful to put you ahead in the prize trade. Most of the time, Darkrai/Hydreigon starts off with Sableye for the first few turns. Sableye gets back crucial Dark Patches and search cards to speed up your setup. Sableye also gets back helpful trainers like Catcher and Max Potion. Once you have enough energy to attack with Darkrai, it is usually best to do just that. Sometimes, though, you may want to just start off with one of your techs. You can then use your techs to respond to whatever your opponent sends out, using weakness to your advantage. Hydreigon's attack is also very useful as it is your only pokemon that can hit for a substantial amount of damage without double damage from weakness.
4 Professor Juniper
3 Max Potion
3 Ultra Ball
3 Rare Candy
2 Random Receiver
2 Dark Patch
With 16 free spaces, there is plenty of room to tune your deck to your own playstyle. There are basically two versions of Darkrai/Hydreigon, Darkrai based and toolbox. The more Darkrai based version focuses on a fast Darkrai to deal heavy damage right from the beginning. This version only runs 1-2 techs, if any. The toolbox version focuses more on techs and manipulating weakness to OHKO anything your opponent brings out. Both of these variants are quite good, and I will explain decklists and matchups for both of them.
This is one awesome card to attack with turn 1. You can get back Dark Patches, Random Receivers, Ultra Balls, and basically any other trainer that you may need. It's an awesome card all around, and the fact that Darkrai/Hydreigon has the fantastic consistency card while other decks don't is a really big boost. While Sableye is amazing turns 1 and 2, it can also be used later in the game to get that clutch trainer that you need to win the game, such as a Max Potion or Catcher. Sableye is easily one of the best consistency cards in the entire format, and running any lower than three would not be good. You really want to maximize your changes of being able to attack with Sableye turn 1.
This card is amazing when paired with Hydreigon's ability. With Hydreigon, you can move all the energy off of a damaged EX, use Max Potion to heal it completely, and then move all the energy back on and continue to attack with it. Max Potion is great for wasting your opponent's hard work, and it is amazing to put yourself ahead in the prize trade. It also allows for a lot of great comebacks, and without Max Potion Darkrai/Hydreigon would definitely not be as powerful as it currently is.
This card is your energy acceleration. It lets you take any basic Dark from the discard and attach it to one of your benched Dark Pokémon. With Dark Patch you can often get Darkrai fully charged up by turn 2, which allows you to start wrecking with his attack a turn earlier. Dark Patch can also be used later in the game to get energy back on the field, which is important in Darkrai/Hydreigon. Because it only attaches to benched Dark Pokemon, you have to be careful that you always keep a Dark pokemon (like Sableye) on your bench so that you can Dark Patch freely (especially if you run the Dragon Deino/Zweilous).