Magic The Gathering: Arena Dev Tracker

They're rare, especially compared to how many players there are overall. I've played basically every day since beta launch, and I've seen ~3 Mythic Orange opponents.

NotVeryGoodAtMagic? Their decks are very brutal, almost cruel.

MTGO's not really my area, so I might be getting this wrong, but I believe the card you're referring to is called Library of Congress

Yup, that's what's up! I explain a bit more above.

In debug modes, we have the ability to directly cheat a bunch of effects (wish for cards, waive the mana for the thing on the stack, bounce a permanent, etc.). It has several options like that, but still leaves a lot that it can't easily do. Back in the early days, I used to have repeatedly cast & bounce Quarry Hauler to test PW abilities. Now we have a large range of debug-only cards to help with situations like this. (For example, one of them says basically "Exile target planeswalker and return it to the battlefield with 50 loytalty".)

Magic is a game complex enough that the cards can make almost anything happen, and we built the Arena rules engine to process all that complexity. So the easiest way to build new debug tools turns out to be basically creating new cards that do what you want.

Heh, I wasn't trying to be mysterious. It's just [[One With Nothing]] and [[Scour from Existence]] (mentioned elsewhere in this thread).

Huh. You weren't supposed to see those. Whoops!

So, most of our regression tests (automated tests to make sure things are working as expected) are written as, effectively, gameplay sequences - Play Runeclaw Bear, play Doom Blade targeting Runeclaw Bear, make sure it dies, etc.

These cards both provided useful effects for those test cases, so we added them to the pool of cards we could use here. They were meant to be hidden, but it looks like the settings to hide them are arriving a bit later than the cards themselves. Don't get too attached; I expect them to disappear soon.