Core 2021 features a mix of new and reprinted cards. The main set contains 274 cards (15 basic lands, 111 common, 80 uncommon, 53 rare, and 15 mythic rare) and includes randomly inserted premium versions of all cards. Teferi, Master of Time (#75) additionally features three alternate arts, numbered #275-277. The Buy-a-Box promotional card is #278. Cards with an alternate card frame have another card number than the original version. Borderless planeswalkers are numbered #279-284, showcase cards are numbered #285-313, alternate art borderless cards are numbered #314-319, cards found in the set's planeswalker decks are #320-339 and the extended artwork cards are numbered #340-391. The Bundle promo is numbered #392, and the dark-frame cards from the universal promo pack are numbered #393-397.
The featured planeswalker is Teferi. The set also features some of his family, friends and foes. A new planeswalker is Basri Ket.
The Showcase card frames are identical to the Signature Spellbook frames for the planeswalkers (cf. Chandra's frame). Teferi, Master of Time features four subtly different artworks. The monocolored planeswalkers each feature three signature spells and a basic land depicting their home plane. Ugin features no additional cards beyond his planeswalker card.