Spacetime Transcendence 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, Spacetime Transcendence is Unlimited.
YGOPRODeck lists Spacetime Transcendence for these formats: Speed Duel, Duel Links, Common Charity, TCG, OCG, Master Duel.
A listed print of Spacetime Transcendence appears in Battles of Legend: Light's Revenge (BLLR-EN029) at Ultra Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Spacetime Transcendence is about $0.03 on Cardmarket (prices change frequently).
