From SC2Pod
Producible once the Spire is evolved into a Greater Spire, the Guardian is morphed from the Mutalisk. Once the transformation is complete, the Guardian form is permanent and the Mutalisk is no longer recoverable.
The Guardian is one of the most powerful Zerg assault units with an incredible attack range and an attack that inflicts a full 20 base points of normal damage. In addition to the awesome attack potential, the Guardian also has 2 points of base armor, making it more resistant to attack than most units.
The Guardian's attack range gives it the advantage of being able to fire on ground targets while hovering in safety over high ground, water or space. The only ground unit truly capable of retaliating at that distance is the Terran Goliath with the Charon Booster Upgrade.
Lore
Mutalisks can also metamorph into another form, which is believed to be based on the nesting form of the Mantis Screamer. The Zerg Guardian has a thicker armored carapace and attacks by lobbing explosive gobs of acid which have a much longer range than the Mutalisk’s spray. This acid can only be directed at targets on the ground below. Guardians have no natural defenses against aerial opponents.
Weapons
Upgrades
StarCraft 2
The Brood Lord's StarCraft 2 model
The Guardian has been replaced by the
Brood Lord in StarCraft 2. The Brood Lord is an even stronger unit, and is similarly only able to attack ground units. Its attacks also spawn
Mantalings, which are a new version of StarCraft 1's
Broodlings.
100 
100 
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