The Elder Scrolls: Legends
There is surprisingly little competition in the area of online card games, the only significant tiles being the digital version of Magic: The Gathering and Hearthstone: Heroes of Warcraft. Since they own a relatively popular fantasy IP, and presumably like money, Bethesda recently decided to throw its hat into the ring and bring us The Elder Scrolls: Legends.
At a casual glance, TES:Legends feels like a straight up clone of Hearthstone, except with a more serious art style as could be expected from The Elder Scrolls. Instead of relying on resource-related cards, players are given a set amount of Magicka that increments on a per-turn basis, which they use to summon creatures, equipment, and spells. Combat mechanics are also very similar. Unlike MTG, creatures controlled by a player can be assigned to attack an arbitrary target under normal circumstances. With rare exception, all player actions take place within their own turns, with no instant-type spells or counters.
Players are guided through a single-player tutorial story which slowly teaches them the mechanics of the game as well as gives them the cards necessary to build a basic deck. There are a large number of cards of varying rarity, and players can earn booster packs by either spending in-game currency, which is earned slowly from playing the game, or by spending real-world cash. There is a limit of three of a specific card per-deck. You can "soul-tap" your excess cards for points which can be used to unlock cards of your choosing, giving you another path to getting the cards you want, though it is a long one. Additionally, the game offers an Arena mode which is similar to a booster draft. Playing in an Arena tournament guarantees you at least one booster pack, potentially more if you do well. As you level up, you also have the opportunity to upgrade certain cards, and can earn extra rewards based on your currently selected avatar's race. For example: if you're working on a dunmer-based deck, then you want a Dark Elf avatar to increase your chances of getting relevant cards.
Despite the major similarities, there are some significant differences between TES:Legends and Hearthstone that become quite apparent. The first is that when building a deck, instead of choosing a single "class" you instead build your deck around two attributes (such as Intelligence, Strength, and so forth) which allows for greater versatility in the types of decks and strategies that can be used. Second, standard combat rules provide two "lanes" upon which creatures may be summoned: The "Field Lane" and the "Shadow Lane." With some exceptions, creatures in one lane cannot attack creatures in the other, and most creatures summoned into the shadow lane cannot be directly attacked for one turn. Choosing which lane to summon your creatures into adds an interesting dynamic to the game's overall strategy that is not present in Hearthstone.
Finally, there is the rune system. Upon being reduced to 25, 20, 15, 10, and 5 health, players expend a rune automatically and draw an extra card. This generally will happen during the opponent's turn, and is the only time the flow the attacker can potentially be broken. If a player draws a card with the "Prophecy" keyword, they may play it instantly without cost. Some prophecy cards include powerful direct damage abilities, others powerful creatures. They do have the potential to turn the tide of the game. Plus the fact that players get cards when they take damage means that if a player does too much damage to their opponent all at once, they might be giving their opponent more ammunition for a counter-attack next turn. In Hearthstone and even MTG, it is often advantages to do as much damage in one turn as possible. However, in TES:Legends players sometimes may choose to holding back in order to prevent their opponent from getting too many cards at once.
Overall, I think The Elder Scrolls: Legends is a decent game. In many ways I believe it offers more variety and strategy than Hearthstone, which it can be easily compared to. The more serious vibe of The Elder Scrolls is a also a nice change of pace if you've had your fill of Blizzard's often over-the-top comedic tone. On the other hand, it also faces the same pitfall as any other collectible card game (physical or virtual), which is that if you want to play it seriously you're going to have to pony up some cash to make sure your deck is in competitive shape. Even though it is still in open beta, I've already run across players who have quite obviously plunked down some change and have gotten some pretty powerful cards as a result. This has lead to some admittedly frustrating matches where it is clear that my meagerly assembled deck is simply outmatched.
Either way, it is a game that I'd recommend at least checking out if you happen to be a fan of both card games and The Elder Scrolls.