Ancient Rules is a Normal 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, Ancient Rules is Unlimited.
YGOPRODeck lists Ancient Rules for these formats: Duel Links, Common Charity, Edison, TCG, OCG, Master Duel.
A listed print of Ancient Rules appears in Champion Pack: Game Seven (CP07-EN019) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Ancient Rules is about $0.02 on Cardmarket (prices change frequently).
