A free to play CCG on kongregate.
The cards are divided into 6 kingdom, 3 dark (undead/alchemist/elemental) and 3 light (holy/ancient/mystic). Each deck is led by your choice of hero (which determines your deck's main kingdom), and can include cards from all kingdoms except your opposing kingdom (undead vs holy, ancient vs alchemist, elemental vs mystic) You are free to switch decks (once you buy the other starters with in game currency), so you do not have to worry about which starter you choose, although alchemist/ancients/holy are easier in my opinion to start with.
Game has PvE campaigns for each kingdom with 3 difficulties. End game goal though is on Pvp and of course obtaining higher rarity cards. The PvE should be used as tutorials really. PvP in the game is divided into several arenas with deck restrictions, so any type of paying/nonpaying players are sure to have fun. For example, bronze arena is restricted to use of only 1 epic and 2 rare cards max in terms of higher rarities and restricted to only use of tier 1 heros.
Gameplay is pretty good, objective is to reduce enemy hero's life to 0. Each hero has their own sets of abilities to help aid you to victory, aside from the hero card there are creature cards (which gets the summoning sickness delay, unless they have haste) spell cards, gear (equipment) cards, and runeword (field) cards. Runeword cards affect both players for a certain amount of turns. Each player starts out with a certain number of health and mana depending on which hero you choose. Each turn players gain 2 mana and draws 1 card. Max hand size is 5 but you are required to draw at the beginning of your turn, so you will end up discarding your rightmost card if you have to draw a card while having 5 cards in hand--This situation should never happen because you are allowed to increase your mana by 1 by discarding a card during your turn (You are allowed to do this as much as you like as long as you have cards to discard) There are 3 slots for creatures on the field and each creature attacks the creature on the opposite slot on the field if there is one, if the opposing slot is empty then the creature will attack the enemy hero even if they have creatures in the other slots. Of course there are many creature abilities that will affect how the creature attacks, etc. There are too many of those, so I'll let you try the game to discover them yourselves.
The game does use an energy system to restrict player progress, however I feel that it is not that bad in this game, since each game costs 3-6 energy depending on which arena you play in, and energy regenerates at a rate of 2 minute per energy. You start out with a max of 50 energy but through progress in the game, you can purchase max energy upgrades. If all upgrades are bought, your max energy can reach up to 125, which is not bad at all. Though only the first 2 upgrades are available for in game currency, the rest are bought via cash currency called gem (you can earn some gems by obtaining achievements in game and by logging in everyday to get daily rewards which sometimes gives gems, in all it will cost about around 14$ worth of gem)
Essentially completely free to play since all the boosters are able to be bought via in game currency (called gold), and the 'grind' if you will for the free players are not extremely discouraging either. Each win can net you around 33-58 gold per energy spent and each loss will net you around 16-17 gold per energy spent. Yes, you get rewarded with gold even if you lose so essentially as long as you play the game you will progress. It is truly a free to play game with paying options to speed up the process because you are not denied of 'any' content (minus 1 mediocre promo exclusive to anyone that spends $10) by not paying, all the cards are available for in game gold, even the promos are sold for gold at a reasonable price.
The game so far has 4 rarities of cards; common, uncommon, rare, and epic. There are currently 4 types of booster packs available, epic, gold, light and dark boosters each contains only 3 cards but the rarity are set in stone so you know exactly what rarity are in your pack. Epic packs are the most expensive at 40,000 gold which is around 3-6 days of saving gold (Depending on how efficient you are with your energy) but it guarantees 1 epic and 2 rare cards, so it is not bad at all. Gold packs are 5,000 gold which guarantees 1 rare and 2 uncommon. Light booster costs 1,000 gold only contains light cards and has 1 uncommon + 2 common, similarly dark booster are also 1,000 gold and contains only dark cards and has 1 uncommon + 2 common.
Singles are available for sale but those are gem purchases only, the singles section are random and refreshes every 15mins but that isn't too bad, you just have to wait for the card you want to show up. The prices are also pretty reasonable with epics being 11$, rares being $4.50, uncommons are $0.50 and commons are $0.10 Actual costs are actually less than that since when you buy kreds you are awarded bonus kreds when you buy in bulk, and then when you buy gems with the kreds on kongregate you are again awarded extra gems if you buy in bulk, so yea, those prices are only rough estimates.