Stern Mentor is a uncommon Creature — Human Wizard card from Avacyn Restored (AVR), collector number 77. Current market price: $0.05 (foil $0.23), TCGplayer market prices via Scryfall.
Soulbond (You may pair this creature with another unpaired creature when either enters. They remain paired for as long as you control both of them.) As long as this creature is paired with another creature, each of those creatures has "{T}: Target player mills two cards."