When it comes into play, it taps down all legendary creatures, and it keeps legends from 'readying' in the untap phase, potentially locking them down for the rest of the match
Not all of them are promo cards and not all of them come with the same criteria
Turn one, draw a card, play nothing, discard a card in the cleanup step, throw Primeval Spawn into the graveyard, turn two basic swamp, cast Reanimate targeting Primeval Spawn is totally something you I would try do in a game of EDH if the restriction wasnt present
A standard Lorcana booster pack will contain 12 cards with one art/marketing card, with the following breakdown per pack: 6 Common cards 3 Uncommon cards 2 Rare, Super Rare, or Legendary cards Both non-foil 1 Foil Card This could be of any rarity from Common to Enchanted 1 Art/Marketing Card That means in one pack you could pull 6 Commons, 3 Uncommons, then 2 Legendary non-foil cards, followed by an Enchanted in the foil slot
A reasonable option, I think Lay Bare the Heart is mostly a modest upgrade over Transgress the Mind (a decent, not great card itself)
That leaves us with 3 good learn cards and another 3 or so that are potentially enough for cube