Join Shields is a uncommon Instant card from Guilds of Ravnica (GRN), collector number 181. Current market price: $0.22 (foil $0.16), TCGplayer market prices via Scryfall.
Untap all creatures you control. They gain hexproof and indestructible until end of turn. (They can't be the targets of spells or abilities your opponents control. Damage and effects that say "destroy" don't destroy them.)