Poisoned Edge:
After this card comes out of shadows, attach it to a character you control. Then, choose and kill an opponent’s character with a printed STR of 2 or less.
Response: After you lose a challenge, Poisoned Edge comes into Shadows.

So what happens when I lose military challenge and I want to kill character that has Poisoned Edge attached to? Will it be discarded or will it make it into shadows?

Yes, you can return poisoned edge to the shadows because the killed character enters the moribound state and doesn't go to the dead pile until alla responses are triggered. So you can declare attached character as the military claim victim and then trigger the poisoned response in step 5 (where responses are triggered).

It's similar to A Lannister Pays His Debts: if the Lanni player loses a military challenge, he can choose to kneel the character/s already chosen as military claim, instead of choosing another character.

I'm not sure...

Moribund state gives you the time to RESPOND, yes, but to the Character entering the discard pile...not to "losing a challenge".

You choose the claim, it enters moribund, respond to him leaving play, then claim is complete. Now, you can respond to the lost challenge...But it's too late, if you chose the attached character as the claim.

IMHO.

Both the character and the attachment will become Moribund. The character will go to the dead pile and the attachment will go to the discard pile. Then Responses can be activated but, since the attachment is to go to the discard pile, you cannot bring it back into shadows:

Although considered in play, a Moribund card cannot be removed from play (or targeted to be removed from play) again by any effect or any attempt to pay a cost for the remainder of the action window. However, the "state" of a Moribund card can be changed by an effect that does not actually attempt to remove it from play a second time.

DB_Cooper said:

There is a timing factor a little off here. Losing a military challenge and kill a character for claim are the first two parts for resolving a challenge. The next two are awarding power for "unopposed" and Renown, is appropriate. You have to do all 4 things before Responses to ANYTHING can be played. So you do not "respond to him leaving play" before claim is complete. The character dies and enters moribund, then you have to wait for the rest of challenge resolution to be complete. All Response opportunities are then simultaneously opened together - and you can play Responses in any order.

Moral of the story: the opportunity to Respond to the character leaving play and to losing the challenge are simultaneous. Response timing is not the issue here.

HOWEVER, both DB and eloooooooi have the situation correct, even if the details for why are not entirely accurate.

When a card leaves play, any attachments on it are discarded passively as a result. This is important because it sets up a separate opportunity to save an attachment. So when the character dies for claim in Step 3 of the challenge resolution window, it enters moribund - but the attachment does not. Interesting situation, where the character is moribund but the attachment is not, right? Well, in Step 4 (the "resolve passives" step), the game takes care of that and discards the attachment from the character. You have an opportunity to save the attachment at this point that is DIFFERENT from the opportunity to save the character from claim in Step 2. (Note that the attachment is in a terminal state, so this save must also move the attachment, return it to hand, or something similar.) If you don't, the attachment enters moribund in Step 4.

Now you get to Step 5, where Responses to everything - like returning the Blade to Shadows after you lose a challenge - can be played. Unfortunately, the attachment is already moribund and cannot be removed from play (into Shadows) a second time.

ok, I got it all wrong... Thanks for the clarification. I believed that the attachment must follow the destiny of the character to which it is attached.