I sent this in a reply to McF4rtson, but I thought I should repost here as it is important for this particular contest:
"until end of turn" is the standard way of writing that phrase in Hearthstone.
Just search "end of turn" in the card list. See Potion of Madness, Shadow Madness, and Power Overwhelming. "At the end of turn" needs "the". "Until end of turn" does not.