The Deadly Keyword in the core rules:

"During a challenge, if the attacking player controls
the most participating characters with the
“Deadly” keyword, the defending player must
choose and kill a defending participating character
after the challenge resolves."

Hypothetical Scenario:

I have Greatjon Umber with Deadly and I have Robert Baratheon(TGM) with Deadly in play.

I attack Player A with Robert Baratheon alone.

Player A defends with a Deadly character but is still losing the combat.

Player B is allowed to declare 1 defending character he controls and chooses to do so with a character that also has Deadly. This is giving Player A enough strength to win the challege.

Player C sees the combat is being won, and does not declare a defender.

I see that I am losing, so I add Greatjon Umber to the combat. I have enough for the win.

So in summary -

I have 2 Deadly characters participating.

Player A has 1 Deadly character participating.

Player B has 1 Deadly character participating.

Per the deadly rules, regardless of me winning of losing the combat, Player A chooses either his participating character or Player B's participating character to die to deadly.

Is this correct?

makes sense to me but im dumb. im still trying to figure out how you gave both of them deadly

Yep, that's correct. The deadly rules require a fairly technical reading when you've got more than 2 players' characters involved in the battle, but your interpretation is spot on.

As to the deadly-granting: Alchemist Shop? Longship Black Wind? Feral Pack with Old Nan making people Direwolves? There are a few ways to do it.

dh098017 said:

It's all hypothetical and most likely too expensive and a pain to pull off, but I was just trying to create a scenario where you would have 2 deadly attacking characters against multiple deadly defending characters controlled by more than one opponent.

Longship Black Wind is one way to give one of them deadly and Fanatic attachment + Black Amethyst attachment is another way I can think of quickly.

radiskull said:

Sweet. Thank you.

I just wanted to make sure that the attacker only had to be tied with one controllers group of Deadly characters participating. In this case, Player B would have negated the Deadly effect by somehow having a 2nd participating character with Deadly that he controlled(if he got the character involved outside "declare 1 defender").

Yep, exactly. You can also end up with weird situations like:

Player A attacks with a deadly dude. Player A also controls Walder Frey.

Player B defends with a stronger deadly dude.

Player A's Walder jumps in on Player B's side.

Player A loses, but controls 2 deadly to Player B's 1. Player B has to kill a dude (probably your Walder, but oh well).

It's worth noting in your scenario, btw, that when you win the power challenge by jumping The Greatjon in, only the original defender has to satisfy claim because Robert it no longer attacking alone.

Here's the thing with Deadly:

1. Figure out which players control participating characters (it doesn't matter which side the character is on)
2. Count up how many of those participating characters - by player - have the Deadly keyword (again, it doesn't matter which side the character is on)
3. If the attacking PLAYER controls the most characters with the Deadly keyword, the defending player chooses and kills a defending CHARACTER (doesn't matter who controls it)

So, as rad implies in his post, if you had jumped your Deadly Greatjon in on the DEFENDING side in your hypothetical, you still would have "won" Deadly.

ktom said:

Even though the other players have declared their 1 defender? I would think that once that point of the challenge has passed, the replacement claim of the challenge is locked in. I tend to agree with you, but I need to play the devil's advocate because it would also be advantageous for the opponents to put Greatjon Umber on your side of a challenge as well(and any other character that can be used like that). That just seems too easy.

Bomb said:

Allowing opponents to declare a single defender each and the claim replacement are two separate effects. One is resolved when defenders are declared, one is resolved when claim is settled. Both are independent of each other, but both are dependent upon Robert attacking alone.