Unending Nightmare is a Continuous Trap Card.
Trap Cards are normally Set first and can be activated on a later turn, often in response to an action.
On the current TCG and OCG Forbidden & Limited lists tracked here, Unending Nightmare is Unlimited.
YGOPRODeck lists Unending Nightmare for these formats: Duel Links, Common Charity, Speed Duel, TCG, OCG, Master Duel.
A listed print of Unending Nightmare appears in Battles of Legend: Relentless Revenge (BLRR-EN104) at Ultra Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Unending Nightmare is about $0.05 on Cardmarket (prices change frequently).
