Jumpstart: Historic Horizons is a MTG Arena exclusive, digital-only set to be released on August 12, 2021, injecting 403 new cards for Historic play.
A special Jumpstart event will run from August 12, 2021 to September 9, 2021. You take two themed boosters (from a total of 46), shuffle them together, and play against your opponent to bring you a mixture of constructed and limited experience.
These are the lists for each of the themed booster packs, which breakdown the cards in each pack and probably of opening each card slot. For more information about this format, check out our dedicated guide.