Archfiend's Oath 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, Archfiend's Oath is Unlimited.
YGOPRODeck lists Archfiend's Oath for these formats: GOAT, OCG GOAT, Speed Duel, Common Charity, Edison, TCG, OCG, Master Duel.
Archfiend's Oath belongs to the Archfiend archetype—use that tag when searching for engine pieces and tech options.
A listed print of Archfiend's Oath appears in Astral Pack Four (AP04-EN024) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Archfiend's Oath is about $0.13 on Cardmarket (prices change frequently).
