When an opponent casts an instant spell, if Hidden Gibbons is an enchantment, Hidden Gibbons becomes a 4/4 Ape creature.
When one of your opponents successfully casts an instant or interrupt spell, if Hidden Gibbons is an enchantment, Hidden Gibbons becomes a 4/4 creature that counts as an Ape.
When these apes want something, it's a matter of gibbon take.
