Nimble Larcenist is the first Obscura card to look at, and while it might seem unassuming, I think its a key piece for blink decks
From a law perspective, yes, custom Magic cards are legal for personal use only
Not only have cards like The One Ring and Orcish Bowmasters completely changed various MTG metagames, but The one of one Serialized Ring turned Magic into a sensation, producing the most expensive MTG card ever, selling for over two million dollars to Post Malone
The effect of a card like this creates cascading damage from creatures that is uniquely resilient against mass removal because you can just keep the damage going on subsequent turns due to haste
For Pokmon trading cards collectors : this Diggersby Reverse 147/192 card is considered as Rare (Carte Pokmon) card
And they are characterized by a unique holofoil pattern