Aven Riftwatcher is a common Creature — Bird Rebel Soldier card from Planar Chaos (PLC), collector number 1. Current market price: $0.37 (foil $0.28), TCGplayer market prices via Scryfall.
Flying Vanishing 3 (This creature enters with three time counters on it. At the beginning of your upkeep, remove a time counter from it. When the last is removed, sacrifice it.) When this creature enters or leaves the battlefield, you gain 2 life.