Flare of Fortitude is a rare Instant card from Modern Horizons 3 (MH3), collector number 391. Current market price: $5.15 (foil $10.16), TCGplayer market prices via Scryfall.
You may sacrifice a nontoken white creature rather than pay this spell's mana cost. Until end of turn, your life total can't change, and permanents you control gain hexproof and indestructible.