The Buy-a-Box promo for Theros Beyond Death includes two cards—one foil Athreos, Shroud-Veiled, and one foil full-art "Nyx" Land, both packaged inside an opaque sleeve. Customers are eligible to get one Buy-a-Box pack for each box of Draft Boosters or Collector Boosters they buy.
However, due to a production error, the lands in these packs are non-foil. A reprint with proper foil treatment is currently underway, and we will ship the replacement lands as soon as possible. The lands will arrive in a separate shipment, after or around Prerelease.
For North America and Japan, this should mean that most stores have the foil Nyx lands available sometime around Prerelease weekend, January 17–19. For other regions, it's likely that the foil Nyx lands will arrive after that date.
Stores will receive the same number of replacement foil Nyx lands as Buy-a-Box packs, so any player who participates in the Buy-a-Box promotion (while supplies last) should also receive a foil Nyx land.
Please make sure to check with your local store to see how they plan to handle redemption of the foil Nyx lands.