The destroyer
The destroyer is a demon. That is the most truthful and blunt way to put it. The destroyer doesn't have emotions and isn't really motivated by anything. In fact, if the one who somond it suddenly cut of the connection, like they died or were magically imprisoned, the destroyer would simply fall apart and it's 'soul', if you can call it that, would return to the underworld. The ancients believed the destroyer would bring about the birth of a new world, by issuing its destruction. The truth of the matter is, it's only purpose is to destroy the world, or bring about the apocalypse. When it is summoned, it emerges from a volcano, and is so big, that it's back is big enough to be classified as a city. It's made out of molten rock and magma.
While it doesn't move its legs too fast, its incredible size means that it takes ridiculously large steps, and thusly moves at a ludicrous speed. To destroy the world, the destroyer leaves a trail of fire in its wake that will never go out. This trail of fire is known as the ring of annihilation, or, the belt of fire. When it is complete(when the destroyer has walked around the entire planet), the destroyer enters the planet through the volcano that it had emerged from, and the ring of annihilation spreads across the surface of the world in a torrent of fire and ash, and is so powerful that it would knock down buildings, destroy mountains, and cause underground caverns to collapse on themselves. Anything less than 1,000 miles below the planet's crust would die. This may or may not also result in the planet breaking into pieces.
The destroyer itself isn't really sentient. It just tries to fulfill its purpose and do its job, what it was summoned for, that is. Because it is made with a mixture of magma and rock, it moves much much slower in the water. In fact, it sometimes gets its feet stuck in a single position. The destroyer also requires some kind of power source, which always ends up being Aether or Nether, but of course this requires that either the destroyer be covered in dark gems, or the one who summoned it needs to continually fuel it nonstop, but since fueling it would require an insane amount of energy, usually the dark gems are used.
