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Whenever an opponent casts a creature spell, if Opal Acrolith is an enchantment, Opal Acrolith becomes a 2/4 Soldier creature.
{0}: Opal Acrolith becomes an enchantment.
A noncreature permanent that turns into a creature can attack, and its {T} 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.
A “creature spell” is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.
Changes into a creature even if the spell is countered.
It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
When it turns into a creature, it is no longer an enchantment.
When it turns into an enchantment, it is no longer a creature.
