The Humble Sentry 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, The Humble Sentry is Unlimited.
YGOPRODeck lists The Humble Sentry for these formats: Duel Links, TCG, OCG, Master Duel.
A listed print of The Humble Sentry appears in Abyss Rising (ABYR-EN065) at Short Print rarity.
Among markets shown on this page, the lowest listed snapshot for The Humble Sentry is about $0.07 on TCGPlayer (prices change frequently).
