Interrupt: When a Scorpion character you control leaves play - attach that character to this holding facedown instead of placing it in its owner's discard pile.
Action: Play an attached facedown character from this holding as if it were in this province. Then, sacrifice this holding.
1. When the interrupt fires, does the character in question leave play? The interrupt clearly applies a replacement effect to the triggering condition "leaves play", but it is unclear what is being replaced. Is the entire act of leaving play being replaced with being attached facedown to the hideaway, or is it only the destination that the character is leaving play into that is replaced?
Questions 2-4 are all assuming the attached character is still in play. If that is not true, feel free to skip to 5.
2. If the character is still in play, what properties does it have? The RRG gives no guidance on how to treat facedown cards that aren't provinces. Is it a character still? Does it have values, traits, a fate cost, etc? If it doesn't have these things, then how can you even play the character with the action?
3. If the character is still in play, what happens to its fate tokens? Status tokens? Attachments? Lasting/delayed effects? It never left play, and so it never went through the process of clearing all of these things, but it also may not even be a character any more. How would Bayushi Shoju's terminal burn effect function on a character saved by Back-Alley Hideaway?
4. If the character is still in play and is still a character, wouldn't the fate phase still apply to it? Should it not lose a fate each round and be discarded if fateless, like every other in play character?
5. If the character is not in play, what happens when you try to use this ability on an opponent's character that you control? For example, a Shosuro Actress of yours brings an opponent's Favored Niece into play under your control and the Niece then leaves play at the end of the round. You attempt to use Back-Alley Hideaway to store the stolen Niece, but that would move it to an out of play state, and per the RRG "If a card would enter an out-of-play area of a player who does not own the card, the card is placed in its owner’s equivalent out-of-play area instead." What is the owner's equivalent out of play state if they don't also have a Back-Alley Hideaway?
6. The character is attached to the Back-Alley Hideaway - can you use Let Go on it? I presume not, because it doesn't appear to be an attachment, but it's been asked anyways.
7. If the character isn't an attachment, what happens when the Back-Alley Hideaway leaves play? The 'Leaves Play' section of the RRG only instructs the player to discard attachments on the card. Does the character remain attached to the Hideaway while the Hideaway is in the discard pile? Can you Rebuild the Hideaway later and play the character? What happens if the Hideaway, with an attached character, gets shuffled into the Dynasty deck? Where should the character be physically located when all of this is happening?
8. Can the action on the card be used during any action window, or only during the Dynasty phase? It says to play the character "as if it were in this province" and normally one can only play characters from provinces during the Dynasty phase. Does that restriction apply to this ability, or does the instruction to do so outweigh that because of the Jade rule?
9. Can the Hideaway's controller look at the facedown cards attached to it? Can the opponent do the same?