Description

Theros is a Magic: The Gathering block, consisting of the expansion sets Theros (September 27, 2013), Born of the Gods (February 7, 2014) and Journey into Nyx (May 2, 2014). The block is a "top-down" design inspired by Greek mythology.

The first set of the block, Theros, was released on September 27, 2013. This large set consists of 249 cards, three of which are Planeswalkers: The white Elspeth, Sun's Champion; the blue/black Ashiok, Nightmare Weaver; and the red/green Xenagos, the Reveler. The set contains five of the block's ten "scry lands," a cycle of dual lands that come into play tapped and allow its controller to Scry 1 when it enters the battlefield. This set also contained a notable reprint in Thoughtseize, previously seen in Lorwyn.

The block introduced several new mechanics and keywords, as well one returning keyword. The following mechanics appear in all sets of the Theros block:

Enchantment Creature and Enchantment Artifact: Cards that are affected by anything that affects enchantments and anything that affects their other type. In addition to having the Enchantment type printed, they are also distinguished by a new starry background representing Nyx.
Heroic: A keyword ability that appears only on creatures. Whenever a creature with Heroic becomes the target of a spell cast by its controller, the ability triggers, causing a one-time effect such as the addition of a specific number of +1/+1 counters or an action that interacts with the opponent's creatures.
Bestow: Bestow is an alternate cost that appears only on Enchantment Creatures. A Bestow creature can be cast for its normal mana cost, or, if cast for its Bestow cost (always more expensive than the converted mana cost), it becomes an Enchantment - Aura spell with Enchant Creature. When attached to a creature, the Bestow card gives the enchanted creature a boost in power and toughness that is equal to the original power and toughness of the Bestow creature, as well as whatever additional abilities and keywords the original had (Flying, Deathtouch, etc.). Whenever the enchanted creature leaves the battlefield, the Bestow card becomes unattached from it and becomes a creature itself.
Devotion: An ability word that checks the number of colored mana symbols among all permanents you control (not including lands) that you have in play. For example, one of the most popular devotion cards of the set, Gray Merchant of Asphodel, has an enters-the-battlefield ability that causes your opponents to lose life, and you to gain life, equal to your devotion to black. Additionally, Theros's pantheon are all played as Enchantments, only becoming creatures if your devotion to their colors has passed a certain threshold.
Scry: A popular keyword that appeared as recently as the Magic 2011 core set, Scry allows players to look at a specified number of cards on top of their library, and choose to put those cards on the top or bottom of their library in the order they choose.
The set also has a unique mechanic that did not appear in Born of the Gods but appears in Journey into Nyx:

Monstrous: an activated ability of creatures that can only be used once per game. When a creature's Monstrous ability is activated, it gains a specified number of +1/+1 counters. Additionally, becoming Monstrous may cause one-time effects on the game state, such as destroying lands or other creatures.