Trail of Mystery is a rare Enchantment card from Duskmourn: House of Horror Commander (DSC), collector number 203. Current market price: $0.27, 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.