Though it does form part of the base set of cards for Shimmering Skies, if you want to see one extra, special version of a card that was first revealed in the Deep Trouble set (though do be warned its a spoiler if youre looking to take on Ursula in Deep Trouble!), click here for further details!
From a playability standpoint, Arthur is probably my favorite card in all of Rise of the Floodborn, as his ability adds a lot of fun combos and mechanics to the game we didnt have in The First Chapter
18's episode of WeeklyMTG to discuss his involvement with WotC on this crossover
While I'm hoping that the return of MSRP for Magic: The Gathering products discourages this, we'll have to wait and see
Starting with a mechanic idea, it felt straightforward to create some cards with the mechanic and/or that supported the mechanic, then add some generic cards like removal spells and simple creatures