Opal Acrolith

The pillow-fort defender that taxes nothing while it sits idle. The design conceit is elegant: as an enchantment it costs an opponent nothing to ignore, occupies no slot in the combat math, and dodges every removal spell aimed at creatures. The trigger flips it into a 2/4 Soldier the instant an opponent casts a creature spell, so the blocker materializes only in response to the threat that justifies it, then sinks back to enchantment for zero mana once the danger passes. That free re-enchant clause is the whole trick: it lets the body duck sorcery-speed creature removal between combats, sidestep board wipes that only catch creatures, and shed summoning concerns at will. The 2/4 frame is deliberately defensive, a wall built to trade up against the small aggressive creatures of its era rather than a body meant to attack. What it really represents is a morph-adjacent idea years before morph existed: a permanent that hides its true form until the moment of use, paying nothing to wait. The cost is reactivity. It only animates on an opponent's creature spell, so against control or noncreature strategies it is a dead enchantment doing nothing, and even against creatures it cannot choose its timing the way a flash blocker can. A clever piece of pre-modern defensive design whose chassis (the toggle between inert permanent and on-demand body) reappears in later sets under cleaner rules.