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Whenever you gain life, put a +1/+1 counter on Trelasarra and scry 1. (Look at the top card of your library. You may put that card on the bottom.)
If you gain life at the same time Trelasarra, Moon Dancer is dealt lethal damage, it dies before its triggered ability can put a +1/+1 counter on it, but you will still scry 1.