I understand a target spell that deals x damage to a creature does not directly decrease its toughness. That said, I was playing a game and one player’s Ryusei, the Falling Star died during his combat phase. When Ryusei dies it deals 5 damage to each creature without flying. Then during his post-combat main phase, he played a sorcery spell that effectively dealt 6 damage to all creatures. I had a 9/9 creature without flying on my board. Would the 5 damage and 6 damage stack up to 11 for that specific turn and kill said creature or would it survive the 5 damage and then survive the 6 damage as well?
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-MadMage
The 6 damage dealt to the 9/9 creature without flying will add up to the 5 damage already marked on it earlier. This is lethal damage, so the creature will be destroyed as a state-based action (C.R. 704.5g).
Remember that in general, damage marked on a creature remains on that creature, effectively, "until end of turn" (C.R. 514.2, 120.6); notably, it doesn't wear off when combat is over. See also this thread, this thread, and this thread.
Using card tags is mandatory in the Magic Rulings forum. Please check your private messages to learn how to use them. I've added them for this post.
-MadMage
Remember that in general, damage marked on a creature remains on that creature, effectively, "until end of turn" (C.R. 514.2, 120.6); notably, it doesn't wear off when combat is over. See also this thread, this thread, and this thread.