Does the deadly trait lead to an additional "death" ?

spiersnv said:

If, after a challenge resolves, the attacking players has more participating characters with "deadly" than the defenders (regardless of who won), the defender must choose and kill one of his/her participating characters.

Note that this happens after claim (if any) is satisfied.

Whether that leads to an "additional death" is going to depend a lot on how the situation is played.

For example, I could simply not defend when you do a Deadly attack. That way, there are no participating defending characters for Deadly to kill - so nothing "additional" dies.

Or against a military challenge with deadly I could defend with 1 low str character who I kill to claim. Then there's no participating defender left to kill for deadly. As KristoffStark pointed out, claim happens before deadly.

does deadly count as a card effect?

Card effect? Yes.

Character ability? No.