Bruna, the Fading Light

2016-07-13 A "when you cast" triggered ability resolves before the original spell resolves. It resolves even if the original spell is countered, and the original spell resolves even if the triggered ability is countered.
2016-07-13 Effects that increase or reduce the cost to cast a spell (such as those of escalate and emerge) don't affect the spell's mana value, so they won't change whether Brisela's last ability restricts that spell from being cast.
2016-07-13 For more information on meld cards, see the Eldritch Moon mechanics article (https://magic.wizards.com/en/articles/archive/feature/eldritch-moon-mechanics-2016-06-27).
2016-07-13 For spells with {X} in their mana costs, use the value chosen for X to determine if the spell's mana value is 3 or less. For example, your opponent could cast Burn from Within (a spell with mana cost {X}{R}) with X equal to 3, but not with X equal to 2.
2016-07-13 In a Commander game, your commander may be Bruna, the Fading Light or Gisela, the Broken Blade, and the other may be in your deck. If they meld into Brisela, Voice of Nightmares, Brisela will also be your commander; but if Brisela leaves the battlefield, only the card chosen as your commander at the start of the game may be put into the command zone.

Card is in preconstructed decks:

mtg.wtf