This is a Horde deck, used for Horde Magic.

For those of you who missed Horde format, it's a co-op format where it's the players vs a deck on auto-pilot.

The players work as a team, similar to Two-Headed Giant. Each play takes their turn simultaneously and have a shared life total, but do not share mana or permanents. The team's life total is the number of players x20. At the start of the game the players get 3 turns of set-up before the Horde  gets a turn. After that the turns go between the players and the Horde .

On the Horde's turn it reveals cards off the top of its deck until it hits a non-token card, which is casts. The Horde has infinite mana of all colors at all times, all of its creatures have haste and attack each turn if able. Any damage the Horde takes is turned into cards discarded from the top of the deck. The Horde loses when it has no cards left in its deck and no cards on the battlefield.

Horde target things at random, so Horde decks should be light or limited on cards that target. If a creature in the Horde has an effect that triggers on dealing damage to a player that player is chosen at random. Horde will cast every card in their hand in a random order, but will only cast a given spell once per phase (so if a creature is bounced back to the hand after being cast in main phase one, the Horde will wait until main phase two to cast it again). Lastly if planeswalkers are in play the Horde will randomly decide each combat phase if it will attack walkers or not. If it does it will send one creature per loyalty counter at the walkers, starting with the largest creatures it has. If the is more loyalty on all walkers than the Horde has creatures, it will randomly choose targets.

There is also the optional Hard Mode. In Hard Mode the Horde gets a Wave every 3 turns. When a Wave hits the Horde reveals the top 5 cards and casts all of them, then reveals until it hits a non-token as normal. This means the Horde could get as many as 6 non-token cards in a single turn (yes, there has been a game against this Horde where it hit Purify, All is Dust, Kozilek the Great Distortion, and both Coaxes in the same wave).

From there each Horde is a little different. Whether or not tokens count as creature cards in graveyards, certain other decisions, etc. For this particular Horde here are the special rules:
-If forced to sacrifice it will always choose the smallest creature first.   
-If will only sacrifice scions to revive Deathless Behemoth, but will do so at every opportunity.
If it hits Pull from the Blind Eternities it will pull Emrakul first, and only pull Kozilek if Emrakul is already in play or in the deck.
-The target of annihilator is chosen randomly.
-Bruna will always target Gissela if available.
-Artisan will target the largest creature available.
-Shrill Howler comes into play transformed as Howling Chorus.
-Token cards in the deck count as cards for card effects. Tokens created by effects are treated as regular tokens.
-Non-token cards in the deck count as lands for cards that mill until you hit a land.
-Mindmelter's effect is sorcery speed, but with infinite mana it will use the ability until all players have no cards in hand. Pray someone held up instant-speed removal.

For those wondering what the design philosophy of a Horde deck is:
60-70 token cards.
Tokens should follow some theme.
Most tokens should be weak (1/1 or 2/2) with some bigger tokens mixed in.
Bulk of the non-token cards should be threats, but not devastating. These are cards that can be disasters if players don't have an answer ready. (For this deck that's cards like Breaker of Armies, Mindmelter, Flayer Drone, etc).
Deck should include some 'bombs', between 5-10, that could decimate the player's strategy. These are cards like All is Dust or Coax from the Blind Eternities for Eldrazi, Zombie Apocalypse or Noxious Ghoul for Zombies, etc.
There should be some 'dud' cards as well. Cards that, while not useless, don't pose a serious threat to players. (For this deck these are Benthic Infiltrator,  Witness the End, Birthing Hulk).