16 cards where the set is “j21” and the cards are can only be conjured in their set

Assault Strobe (Jumpstart: Historic Horizons #786)
Dark Ritual (Jumpstart: Historic Horizons #783)
Duress (Jumpstart: Historic Horizons #784)
Fog (Jumpstart: Historic Horizons #789)
Force Spike (Jumpstart: Historic Horizons #780)
Giant Growth (Jumpstart: Historic Horizons #790)
Kraken Hatchling (Jumpstart: Historic Horizons #781)
Lightning Bolt (Jumpstart: Historic Horizons #787)
Light of Hope (Jumpstart: Historic Horizons #777)
Ponder (Jumpstart: Historic Horizons #782)
Reassembling Skeleton (Jumpstart: Historic Horizons #785)
Regal Force (Jumpstart: Historic Horizons #791)
Shivan Dragon (Jumpstart: Historic Horizons #788)
Stormfront Pegasus (Jumpstart: Historic Horizons #778)
Swords to Plowshares (Jumpstart: Historic Horizons #779)
Tropical Island (Jumpstart: Historic Horizons #792)

The message was clear to Sorin—time to search for something else.