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A Gigas Shadow has a vaguely humanoid body structure. It has a round, spherical head with circular, glowing yellow eyes. It also has two long, twisted antennae sprouting out of the top of its head. Each of its hands has three clawed fingers, and its feet are large and lack any distinguishable digits. With the exception of its eyes, this Heartless's body is completely pitch-black.
The Gigas Shadow is identical to the basic Shadow and Mega-Shadow Heartless, but is much larger than either of its two relatives. In Kingdom Hearts Final Mix, they are simply normal sized Shadows that gradually grow to their limit, possibly a reference to Sora's use of the shrinking potion.
Kingdom Hearts Final Mix
Gigas Shadows only appear after three waves of Shadows have been defeated in the Bizarre Room. They attack the same way normal Shadows do, but they do not become flat and move along floors. They are large and have much more HP than other Heartless in the area. They can quickly surround and swipe at Sora, but this can be avoided by moving into a corner on an elevated surface where only a few Gigas Shadows can attack at once. If a Gigas Shadow manages to strike Sora, even just once, it will escape the fight, reducing the odds of winning a Fury Stone.
Using Dumbo will prevent the Gigas Shadows from escaping, though he does little damage. Aeroga and the slightly less effective Aerora can also be used to deflect oncoming attacks. Long-range techniques, like Blizzaga and Strike Raid, can deal damage while keeping Sora away from the Gigas Shadows. Alternately, cast Stopga to freeze the Gigas Shadows and attack them with Sonic Blade or Strike Raid.
Gigas Shadows are relatively easy to defeat. They are only capable of a claw attack, a jump attack, and becoming flat and moving along the floor. Blocking a claw attack will do nothing but blocking a jump attack will stun them temporarily, giving Roxas time to do a combo. Repeat this enough times and the Heartless will fall easily.
The Gigas Shadow's name refers to its massive size—"gigas" is an Ancient Greek word meaning "giant"—and its otherwise identical appearance to a normal Shadow.