When an opponent casts an enchantment spell, if Hidden Ancients is an enchantment, Hidden Ancients becomes a 5/5 Treefolk creature.
10/4/2004 When it turns into a creature, it is no longer an enchantment.
10/4/2004 It changes into a creature even if the spell is countered.
8/1/2008 A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.