Dark Core 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, Dark Core is Unlimited.
YGOPRODeck lists Dark Core for these formats: GOAT, OCG GOAT, Duel Links, Common Charity, Edison, TCG, OCG, Master Duel.
A listed print of Dark Core appears in Dark Revelation Volume 1 (DR1-EN090) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Dark Core is about $0.09 on Cardmarket (prices change frequently).
