Sage of Days is a common Creature — Human Wizard card from The Lost Caverns of Ixalan (LCI), collector number 73. Current market price: $0.10 (foil $0.09), TCGplayer market prices via Scryfall.
When this creature enters, look at the top three cards of your library. You may put one of those cards back on top of your library. Put the rest into your graveyard.