- Although the wording is somewhat unusual, it works just like any other "do X to do Y" ability, i.e. you sacrifice Roose and choose targets first, and then your opponent can cancel the kill effect with Treachery. (Source)
Ally. House Bolton. Lord.
Reaction: After you win a challenge in which Roose Bolton is attacking, choose up to X total STR worth of characters controlled by the losing opponent. X is Roose Bolton's STR. Sacrifice Roose Bolton to kill each of those characters.