They are a silver-bordered, double-sided Princess Luna/Nightmare Moon Magic card, a silver-bordered Princess Twilight Sparkle Magic card, and a silver-boarded Rarity Magic card
Also similar to lands, one of the big reasons that Commander 2018 was so poorly received is that the decks were severely lacking in good colorless cards (in fact, for half of the 2018 decks, the only colorless card worth mentioning was Sol Ring, which has been in every Commander deck since the start of the series)
In short: All cards released on MTG Arena are legal in Historic, except those that were banned from the format or cards that werent ever legal in the first place
Reading MaRos comments over the weekend, I still agree with him that the playtest cards are problematic
Lurrus of the Dream-Den Lurrus of the Dream-Den is my number 10 most broken Magic card, largely due to the Companion mechanic