Onslaught of the Fire Kings 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, Onslaught of the Fire Kings is Unlimited.
YGOPRODeck lists Onslaught of the Fire Kings for these formats: Duel Links, Common Charity, TCG, OCG, Master Duel.
Onslaught of the Fire Kings belongs to the Fire King archetype—use that tag when searching for engine pieces and tech options.
A listed print of Onslaught of the Fire Kings appears in Hidden Summoners (HISU-EN055) at Super Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Onslaught of the Fire Kings is about $0.16 on Cardmarket (prices change frequently).
