First game I played with the new team builder system (although I can't think how this would effect Karthus). Through out most of the game Karthus's ult dealt so little damage I hardly noticed this until the point where I thought it would kill me but when I go back to the replay I find that it had literally done 0 damage to me but it had dealt damage to other members of my team. I was playing as Lee Sin, the Renekton in my game also claimed to have taken no damage. I have the replay but I am not sure how to add it.
Edit 1: I rewatched the replay it would appear that the damage was meant to go through but Karthus did die very soon after perhaps there is a timing issue with the animation.
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