Oko, Thief of Crowns and other banned cards will be back but not as overpowered as before.
Magic: The Gathering Arena is bringing back banned cards like Oko, Thief of Crowns and Teferi, Time Raveler in an upcoming digital event next month. As part of the Summer of Legend announcement, Wizards of the Coast announced that an MTG Arena event called Mirror, Mirror, a mode where players can use banned cards in rebalanced forms.

“Imagine a world where Oko, Thief of Crowns was printed with balanced abilities and became just another great Historic card,” WotC said . “What if Teferi, Time Raveler was printed where he was more of a friend than foe to the format? In Mirror, Mirror, you’ll explore this other universe of decks featuring rebalanced versions of some of Historic’s iconic banned cards. This legendary event won’t stay around for long, but you’ll just have to wait and see what’s inside later this year.”
WotC hasn’t revealed how exactly they are rebalancing these banned cards yet but expect them to be less broken, their mana cost will probably be increased a bit, and we might see some of their abilities adjusted to make sure they’re not overpowered. It’s likely that we will see more banned Historic cards rebalanced in this event. Cards like
Once Upon a Time
Once Upon a Time
If this spell is the first spell you've cast this game, you may cast it without paying its mana cost.
Look at the top five cards of your library. You may reveal a creature or land card from among them and put it into your hand. Put the rest on the bottom of your library in a random order.
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Veil of Summer
Veil of Summer
Draw a card if an opponent has cast a blue or black spell this turn. Spells you control can't be countered this turn. You and permanents you control gain hexproof from blue and from black until end of turn. (You and they can't be the targets of blue or black spells or abilities your opponents control.)
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Uro, Titan of Nature’s Wrath
Uro, Titan of Nature’s Wrath
When Uro enters, sacrifice it unless it escaped.
Whenever Uro enters or attacks, you gain 3 life and draw a card, then you may put a land card from your hand onto the battlefield.
Escape—{G}{G}{U}{U}, Exile five other cards from your graveyard. (You may cast this card from your graveyard for its escape cost.)
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Winota, Joiner of Forces
Winota, Joiner of Forces
Whenever a non-Human creature you control attacks, look at the top six cards of your library. You may put a Human creature card from among them onto the battlefield tapped and attacking. It gains indestructible until end of turn. Put the rest of the cards on the bottom of your library in a random order.
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Nexus of Fate
Nexus of Fate
Take an extra turn after this one.
If Nexus of Fate would be put into a graveyard from anywhere, reveal Nexus of Fate and shuffle it into its owner's library instead.
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Field of the Dead
Field of the Dead
This land enters tapped.
{T}: Add {C}.
Whenever this land or another land you control enters, if you control seven or more lands with different names, create a 2/2 black Zombie creature token.
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Wilderness Reclamation
Wilderness Reclamation
At the beginning of your end step, untap all lands you control.
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Omnath, Locus of Creation
Omnath, Locus of Creation
When Omnath enters, draw a card.
Landfall — Whenever a land you control enters, you gain 4 life if this is the first time this ability has resolved this turn. If it's the second time, add {R}{G}{W}{U}. If it's the third time, Omnath deals 4 damage to each opponent and each planeswalker you don't control.
might return with adjustments, so this should be exciting for players who enjoyed playing these overpowered cards.
Related:
MTG Arena’s Mirror, Mirror event will take place in July, and expect Wizards to reveal more information about the event this month.