Magnum Shield is a Equip Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Magnum Shield is Unlimited.
YGOPRODeck lists Magnum Shield for these formats: Duel Links, Common Charity, TCG, OCG, Master Duel.
A listed print of Magnum Shield appears in 2014 Mega-Tin Mega Pack (MP14-EN035) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Magnum Shield is about $0.18 on Cardmarket (prices change frequently).
