Veiled Serpent is a common Enchantment card from Urza's Saga (USG), collector number 110. Current market price: $0.18, TCGplayer market prices via Scryfall.
When an opponent casts a spell, if this permanent is an enchantment, it becomes a 4/4 Serpent creature with "This creature can't attack unless defending player controls an Island." Cycling {2} ({2}, Discard this card: Draw a card.)