Demon Tower

The Demon Tower is a Pureblood Heartless which serves as a boss in Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-, and Kingdom Hearts III. It is a mass of Shadows in a twister formation native to the Realm of Darkness.

Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-
Aqua first encounters a Demon Tower in the remains of the Castle of Dreams after activating five Gears to restore the bridge to the castle, and swiftly defeats it after a battle. Later, after her encounter with Terra and Ventus in the Forest of Thorns, she reunites with King Mickey in the Depths of Darkness, where another Demon Tower ambushes the two Keyblade Masters.

Kingdom Hearts III
A year later, Mickey and Riku trace Aqua's steps to the Dark Margin, where they are attacked by a Demon Tower. During the ensuing fight, the Demon Tower swallows and spits out Riku, giving him a haircut in the process, damages Mickey's Kingdom Key D, and breaks Riku's Way to the Dawn in half, forcing them to retreat to the Mysterious Tower to restock on supplies.

When the two return, Anti-Aqua has another Demon Tower restrain Mickey before revealing herself to the two. She sends the Demon Tower to attack Riku, but with Sora's help, Riku is able to destroy the Demon Tower.

At the Keyblade Graveyard as part of a final test for the Guardians of Light, Dark Riku summons a legion of Demon Towers followed by an army of Heartless, Nobodies and Unversed.

Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-
The Demon Tower is encountered thrice in the game: the first in Castle Town, the second in the Depths of Darkness, and the third in a boss rush in the World Within. Despite the differences in health throughout the fights and King Mickey's presence in the second, all three fights play similarly. It only has a couple of attacks, indicated by the Shadows' eyes glowing red: one in which the Tower rams Aqua, which can be defended by Barrier, and another in which the Tower divides into the Shadows. When the Tower is separated, the Shadows can be attacked and destroyed. Shotlocks, Situation Commands, and magic can help subdue the Tower quite quickly.

Kingdom Hearts III
The Demon Tower behaves the same way in this game, except during the second fight, Anti-Aqua occasionally comes out and attacks.

The Demon Tower can be fought again by accessing  in Monstropolis. In this fight, it is flanked by several Flame Cores, but it otherwise behaves the same way as the previous fights.