Trail of Mystery is a rare Enchantment card from Khans of Tarkir (KTK), collector number 154. Current market price: $0.22 (foil $0.90), TCGplayer market prices via Scryfall.
Whenever a face-down creature you control enters, you may search your library for a basic land card, reveal it, put it into your hand, then shuffle. Whenever a permanent you control is turned face up, if it's a creature, it gets +2/+2 until end of turn.