Cards launched on special occasions or as part of partnerships Splinterlands has with other entities are called promo cards.
They usually come in a very limited supply by design and some of them are valued very well on the secondary market.
Promo cards also offer a 10% bonus when played in ranked battles, if they are not included in a main edition, like Doctor Blight is in Chaos Legion.
See also Essence Orb mini-edition/Promo Cards.
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Splinterlands Website
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