I am attack with Sapling of Colfenor. Second ability triggers, and on its resolve the top card of my library is Serra Avatar. My life total is 10. Did i actually gain 10 life and lose 20? Or +10 and -10?

You follow the instructions in the order they're printed on the card. First, we look at the creature's toughness (which is 10), and then you gain that much life, putting you at 20. Then we check the creature's power (which is 20, since that's your life total now), and then you lose that much life, putting you at 0 life. Then you put it into you hand. And then you likely lose the game since you're at 0 life now.

Nathan Long
Wizards.Com Boards NetRep