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This was a limited edition product that only 12,000 were produced. The War of the Spark Mythic Edition contains 24 sealed packs, 8 of which will come with one planeswalker card inside the pack (plus the normal rare or mythic card).
The eight special packs will each come with one of each planeswalker in an extended alternate art:
Jace, the Mind Sculptor
Ugin, the Spirit Dragon
Tezzeret the Seeker
Garruk, Apex Predator
Nicol Bolas, Dragon-God
Nahiri the Harbinger