I'm just looking for a definitive answer as to whether the discard from this card is part of the effect or the cost. It seems an argument can be made that it is the cost of whatever effect happens, and this is how I used to think it worked. However, after some thought (instigated by obtaining Corpse Lake, where it actually makes a difference), I believe it is the event's effect, partly because you can't pay costs with opponents' cards and partly because it says "choose an opponent who must discard (however many cards they choose), and then based on their choice…" and not "an opponent must discard … to cause one of three effects". Anyway, I think I already have my answer, but I wanted confirmation.

It's the effect. Your opponent simply gets to choose how many and which bonus you get.

Remember that costs always appear as X in "Do X to do Y." Called to Arms doesn't look anything like that.

It's also important to realize that what you get to do is based on how many cards you opponent chooses to discard, not on how many cards he actually discards. For example, your opponent can have 1 cards left in his deck and "choose" to discard 3. He still discards the single card, but your characters get the +1 STR and Deadly, they do not stand, because he chose to discard 3.

So, since it is the choice of the number, not the actualy number of cards that hit the discard pile, that matters, there is no way the discarding can be a cost.

The choice isn't even a cost. It's an effect, which doubles as a play restriction/condition on the rest of the resolution.

Yeah, that's what I thought, just wanted to be sure. It's also worth noting (on the point of the choice mattering, not the actual number of cards being discarded) that the same holds true if something - namely, the Motley Crewman - forces them to discard more than they choose. The card specifically says, "If they choose to discard… do [whatever the case may be]". Thanks for the responses.

(It's also worth noting that if discarding the cards was a cost, Motley Crewman wouldn't add to the discarded card count.)

Yeah, same idea. It was actually Motley Crewman's interaction with it that first made me analyze the card more closely. I was playing a game and my opponent proposed that he should have to discard extra cards after making hs choice.