Secret Lair will be limited edition box sets available for $29.99-$39.99 for only 24 hours each on https://secretlair.wizards.com/.

They will include a code for a set of the cards on MTGO as well as a sleeve of one of the card arts for use on MTGA.

Their availability will be as follows:

  Secret Lair Bundle (all seven drops) - 12/2, 9am PT, $199.99
  Bitterblossom Dreams - 12/3, 9am PT $29.99
  Eldraine Wonderland - 12/4, 9am PT
  Restless in Peace - 12/5, 9am PT
  Seeing Visions - 12/6, 9am PT
  <explosion sounds> - 12/7, 9am PT
  Kaleidoscope Killers - 12/8, 9am PT
  OMG KITTIES! - 12/9, 9am PT, $39.99