The Zaishen Medallion is a collectable item giving when you win in PvP during the Guild Wars Nightfall PvP Preview Weekend. It is not confirmed if this is a permanent addition or just for the weekend. You take the Medallion to the Zaishen Supplymaster in the Great Temple of Balthazar. If you collect 30 medallions, you can exchange them for either Creme Brulee or a Flask of Firewater. If you collect 50 medallions, you can get a Flame of Balthazar. When you take 10 of those flames to the next supplymaster, you can get a Celestial Sigil. That is 500 medallions needed to get the sigil.
You get 5 medallions if you win normally, +1 medallion for a flawless victory, and +5 medallions for each 5 consecutive wins.