Burden of Proof is a uncommon Enchantment — Aura card from Murders at Karlov Manor (MKM), collector number 42. Current market price: $0.12 (foil $0.04), TCGplayer market prices via Scryfall.
Flash Enchant creature Enchanted creature gets +2/+2 as long as it's a Detective you control. Otherwise, it has base power and toughness 1/1 and can't block Detectives.