Pendulum Impenetrable is a Quick-Play 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, Pendulum Impenetrable is Unlimited.
YGOPRODeck lists Pendulum Impenetrable for these formats: Duel Links, Common Charity, TCG, OCG, Master Duel.
A listed print of Pendulum Impenetrable appears in Legendary Dragon Decks (LEDD-ENC18) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Pendulum Impenetrable is about $0.04 on Cardmarket (prices change frequently).
