With the obvious exception of the R&D playtest cards (they all say not for constructed play at the bottom), all Mystery Booster cards are legal for Constructed play
Occasionally, people experiment with variants like spinbet login in their search bars, or even less precise versions
There are a couple different ways to obtain them, and they can be spent on on a number of things within MTG Arena, including booster packs and cosmetics in the Store
2 Fires Of Invention Was Easy To Work Around Fires of Invention tried to balance its powerful effects (casting spells for free if a player's mana cost was less than their number of Lands) with its powerful drawbacks (a maximum of two cards could be cast in any way, and the player could only cast spells during their turn)
Some necessary backstory, and consider this your FF7 spoiler alert: After the burning of Nibelheim years before the primary events of the game, Zack and Cloud (along with Tifa) are gravely wounded in the confrontation with Sephiroth
A staple of Rebel, white weenie, and replenish decks