Valhalla, Hall of the Fallen is a Continuous 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, Valhalla, Hall of the Fallen is Unlimited.
YGOPRODeck lists Valhalla, Hall of the Fallen for these formats: Duel Links, Speed Duel, Common Charity, Edison, TCG, OCG, Master Duel.
A listed print of Valhalla, Hall of the Fallen appears in Battle Pack 3: Monster League (BP03-EN154) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Valhalla, Hall of the Fallen is about $0.08 on Cardmarket (prices change frequently).
