Soul of Fire 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, Soul of Fire is Unlimited.
YGOPRODeck lists Soul of Fire for these formats: Duel Links, Edison, TCG, OCG, Master Duel.
A listed print of Soul of Fire appears in Force of the Breaker (FOTB-EN031) at Super Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Soul of Fire is about $0.51 on TCGPlayer (prices change frequently).
