Core 2021 features a mix of new and reprinted cards. The main set contains 274 cards (111 commons, 80 uncommons, 53 rares, 15 mythic rares, 15 basic lands) 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.[4] 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).[5]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.
Core Set 2021 is closely tied to Jumpstart, a booster product that released two weeks later.[12][13][1]Jumpstart is meant to provide an easy starting point for players who don't have the appetite or time for deckbuilding. Jumpstart shares 120 cards with Core set 2021.
Tokens and emblems
Core 2021 comes with 20 tokens and emblems.[14] Token and emblems appear in the marketing card slot. The last two tokens are available from collector boosters only.
The mill mechanic now officially becomes a keyword action.[16] For a player to mill N, they put the top N cards from their library into their graveyard.
Five uncommon legendary Shrineenchantments, each of which has an effect that scales with the number of Shrines you control. There is a sixth five-color Shrine printed at rare, Sanctum of All. All of them are illustrated by Johannes Voss.
Five mythic rareplaneswalkers each head a vertical cycle of cards referring to them that also synergize with each other. Each of these cards is also printed in a Signature Spellbook-esque alternate card frame, themed to that respective planeswalker. Chandra's cards match her Signature Spellbook frame exactly.[5]
Ten common lands that enter the battlefield tapped, gain the controller one life, and can tap for two different colors of mana. Reprinted from Khans of Tarkir .
Five uncommon creatures, included as three-ofs in their respective decks. Each interacts directly with – or in one case, supports the theme of – their respective planeswalker.
Five common creatures, included as four-ofs in their respective decks. Each supports the theme of – or in one case, interacts directly with – their respective planeswalker.
Ugin, the Spirit Dragon was introduced to Arena for the first time in Core Set 2021. It gained a reputation in Standard and Historic as a "get out of jail free" card that would annihilate opponents' boards and linger as a growing threat, but such effects are generally balanced as an eight mana spell. It was not its effects, however, but its colorless identity that led to its ban in Historic Brawl, a format where decks are defined by their color restrictions. This ban occurred concurrent to the bans of Agent of Treachery and Field of the Dead, which were already banned in several other formats, for Wizards stated that "each one either exploits or pushes against one of the primary elements of the format".[21]
The Arena Starter Kit (in previous editions known as the Spellslinger Starter Kit) comes with two Standard-legal 60-card decks, designed to create an interactive learning experience for two.
4/4 Angel creature with flying for Angelic Ascension and Speaker of the Heavens.
5/5 Demon creature with flying for Archfiend's Vessel.
1/1 Goblin Wizard creature with prowess for Goblin Wizardry.
3/3 Beast creature for Elder Gargaroth and Garruk, Unleashed.
*/* Weird creature for Experimental Overload.
4/4 Construct creature for Chrome Replicator
: Flying matters
MM.
MM or 
MM and tutors for a specific planeswalker.