The way it works, is when Thassas Oracle enters the battlefield, you get to look at the top X cards of your library, where X is equal to your devotion to blue (how many blue mana symbols appear in the casting cost of all permanents you control)
Ygra, Eater of All Bloomburrow s Ygra, Eater of All has proven to be a ferocious threat in Pioneer, slotting into Jund Sacrifice decks to provide Cauldron Familiar with all the fuel it could desire
Victims are lucky if they remember ever having had what was lost
That is a pretty expensive, but the mythic rarity combined with the potential implications on Pioneer are definitely going to put this card in high demand until players figure out how real its threat is
Moving cards from those zones has different terms, like milling from the library or sacrificing from the battlefield
Lets say you want to be a bit mean at your table