1) Say a character has Venomous Blade attached and an opponent steals that character with Seductive Promise.

Seductive Promise would transfer control of the character but not control of Venomous Blade. Would VB be immideately discarded because of the "After Venomous Blade comes out of Shadows, attach it to a character you control." text? Or is that only a coming into play from shadows restriction?

2) House X only.

Is that strictly a deckbuilding restriction or is that an in play restriction also? If it is a in play restriction would the card be discarded if someone of a non matching house took control of it?

Darksbane said:

This is a little tricky. Because the "attach to a character you control" is part of the passive effect, it is a play restriction on that effect, not an attachment restriction on the attachment itself. So the Blade would not be discarded as illegal if the owner lost control of the character. This sort of thing is perhaps easier to see on a card like "He Calls It Thinking," where attaching to a Martell character is part of the resolution of the Response rather than an attachment requirement - meaning that the "+2 STR Boon attachment" the event becomes can be moved to a non-Martell character or would not be discarded if the original character somehow lost the Martell affiliation.

The reason this is tricky is because if the "character you control" text was an attachment restriction rather than a play restriction on the passive effect, (like on Price of Nobility or Hrakkar Pelt) it would be discarded if you lost control of the character. The situations are different, but in a many ways, it feels like they shouldn't be.

Darksbane said:

Strictly a deck-building limitation. Assuming the deck is legal, it means nothing during the game. Unless, of course, a card effect refers to it (like the gold penalty from the City of Shadows Agenda).