Alara Reborn is the third set of the Alara block, and it’s a special one like nothing else before. This is because all cards in the set are multicolored, and there are no lands - just pure gold stuff around. The set also brings one of the most unfair abilities ever conceived: cascade. Cascade is a triggered effect that when the spell is cast, it lets you exile cards from the top of your library until you find one nonland card that costs less than the original spell, then put the found card onto the stack without paying for it. A clear example is Bloodbraid Elf, the flagship of Jund decks in Modern that costs four mana. When you play this card, you exile cards until you reveal something that cost three or less, like a Tarmogoyf or Liliana of the Veil, and then you can play it for free. The ability also happens if the original spell is countered, because you just need to play it.
The Bloodbraid Elf is not the only most played card in the set, as there are also many other famous ones. A few examples include Maelstrom Pulse, the one that destroys target nonland permanent and all the others with the same name, Sphinx of the Steel Wind, a huge flyer with various protections and lifelink, Anathemancer, the plague of nonbasic lands, Dauntless Escort, the friend of aggro decks at that time which sacrifices itself to make creatures indestructible, as well as a reprint of Meddling Mage.
Other than this lot are the two cards Sovereigns of Lost Alara and Finest Hour, which played key roles in the tournament scene at that time with the infamous Bant Conscription deck. The deck used the exalted trigger of Sovereigns of Lost Alara to get an aura spell and attached it to the attacking creature, together with Finest Hour, the only exalted that allows the attacker to do it twice. If the selected aura is an Eldrazi Conscription, you will have around 20 damage while the opponent will have to sacrifice 8 permanents.