Serra Ascendant says “As long as you have 30 or more life, Serra Ascendant gets +5/+5 and has flying.” That’s a static ability that creates a continuous effect when it meets its conditions. A static ability is always checked, doesn’t use the stack and therefore cannot be responded to. Now that you know this, can you guess why Serra is not dead?
Well, there’s one piece of the puzzle you’re missing: State-Based Actions. It is a “checklist” the game does whenever a player would receive priority. One of them says that if a creature has lethal damage marked on it, it is destroyed.
There’s just one more piece of the puzzle — how the combat damage step works. After both players successfully pass priority in the blockers step, the game now moves on to the damage step. The players assign their damage: In this case, he’ll assign one damage on your cat and you’ll assign two on his monk. The damage is now dealt: two damage is marked on the Serra Ascendant and one damage is marked on Wild Nacatl.
But (there’s always a but!), Serra has Lifelink. Its controller gains one life at the exact moment damage is dealt and is now at 30 life. Serra’s static ability is now active, and it creates a continuous effect granting him flying then modifying its P/T to 6/6 with two damage marked on it. After all this, priority would be given to your opponent, so State-Based Actions are checked. They see that a Wild Nacatl is 2/2 with one (non-lethal) damage marked on it, and a 6/6 Serra Ascendant with two (also non-lethal) damage marked on it. Both creatures survive, and you now understand why he made such a “bad move” by attacking!
Today’s Rules Tip written by Samuel Tremblay