Bloodbraid Elf

• WOTC Official Reminder Textofficial rules
  Cascade — When you cast this spell, exile cards from the top of your library until you exile a nonland card with a lesser mana value. You may cast it without paying its mana cost. Put the exiled cards on the bottom of your library in a random order.

• WWizards of the Coast2021-06-18
  Official Ruling
  If the card has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
• WWizards of the Coast2021-06-18
  Official Ruling
  The mana value of a split card is determined by the combined mana cost of its two halves. If cascade allows you to cast a split card, you may cast either half but not both halves.
• WWizards of the Coast2021-06-18
  Official Ruling
  You exile the cards face up. All players will be able to see them.
• WWizards of the Coast2021-06-18
  Official Ruling
  Due to a 2021 rules change to cascade, not only do you stop exiling cards if you exile a nonland card with lesser mana value than the spell with cascade, but the resulting spell you cast must also have lesser mana value. Previously, in cases where a card's mana value differed from the resulting spell, such as with some modal double-faced cards or cards with an Adventure, you could cast a spell with a higher mana value than the exiled card.
• WWizards of the Coast2021-06-18
  Official Ruling
  If a spell with cascade is countered, the cascade ability will still resolve normally.
• WWizards of the Coast2021-06-18
  Official Ruling
  If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those to cast the card.
• WWizards of the Coast2021-06-18
  Official Ruling
  Cascade triggers when you cast the spell, meaning that it resolves before that spell. If you end up casting the exiled card, it will go on the stack above the spell with cascade.
• WWizards of the Coast2021-06-18
  Official Ruling
  A spell's mana value is determined only by its mana cost. Ignore any alternative costs, additional costs, cost increases, or cost reductions.
• WWizards of the Coast2021-06-18
  Official Ruling
  When the cascade ability resolves, you must exile cards. The only optional part of the ability is whether or not you cast the last card exiled.

Rulings via Scryfall

• WO
  Wizards of the Coast2013-01-28

  Bloodbraid Elf was banned in modern on January 28, 2013. Jund has been the most successful deck at high-level tournaments, and its dominance is making the format less diverse overall. The DCI wanted a card that top players consistently played four copies of in Jund, but ideally was less played in other top Modern decks, and Bloodbraid Elf best fits this criteria to create a more balanced metagame. Source: https://magic.wizards.com/en/news/january-28-2013-dci-banned-restricted-list-announcement-2013-01-28
• WO
  Wizards of the Coast2015-02-03

  Bloodbraid Elf was banned in modern on February 3, 2015. This was the time of Jund, considered by many to be the best deck in the format and not easily addressed. Jund took six of the Top 16 decks at Bilbao. Bloodbraid Elf was the first card from Jund to get the axe. Source: https://magic.wizards.com/en/news/arcana/brief-history-modern-banned-list-2015-02-03