In games where you dont have enough lands or the right colors to cast them, they fix that problem
(One preposterous example: a creature that cant be blocked as long as the spellcasting player is wearing denim.) See also: 'Magic: The Gathering Arena' Closed Beta Play Begins Now First came 1998s Unglued, then 2004s Unhinged
Then, select the deck you want to personalize, and when editing that deck, go to More Details, to the right: Notice that sometimes you may miss and to the Sideboard (which is right above More Options), but what youre looking for is the screen shown below: Within a deck's details, you can set individual preferences for companions, sleeves and avatars for when you play that deck
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A Temur-based Elemental Dinosaur, Kalamax encourages its controller to build a deck heavily around instants, as whenever its controller casts their first instant spell each turn, if Kalamax is tapped, they get to copy that spell and choose new targets for that copy
Gargos still remains relevant even once you've assembled your mountain of mana, thanks to its second ability