RRG "Ownership and Control"
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. (For all associated card ability and framework effect purposes, the card is considered to have entered that opponent's out of play area, and only the physical placement of the card is adjusted.)
Reaction: After a character is placed in your discard pile, move it to your dead pile.
Player A plays Ward, steals player B's character, then discards it to Marched to the Wall. Who can trigger their Nagga's Ribs, player A or player B?
Reading the RRG, it seems that the stolen character is considered to enter player A's discard pile, so player A can trigger Nagga's Ribs, even though that character is never physically placed into player A's discard pile. Is this correct? So basically there is no functional difference between "after a character you control is discarded" and "after a character is placed in your discard pile" in regard to "take control" effects, yes? Same question also applies to And Now My Watch Begins.
Also, if I steal two of my opponent's characters and they're both killed at the same time (due to Valar Murghulis, also House of the Undying), who chooses the order in which they are placed in the dead pile? Since they are considered to enter my dead pile, should I arrange them? Or should their owner do it, as per RRG "Dead Pile"?