Dive to the Heart

The Awakening or Dive into the Heart is a location which appears in Kingdom Hearts Birth by Sleep, Kingdom Hearts, Kingdom Hearts II and Kingdom Hearts Coded. It is framed as a world within a dream, though the events which occur within it have a real effect on the waking world, and in one instance they are actually observed by outside forces. In terms of gameplay, the purpose of the Awakening is to serve as a tutorial level, where the player will learn the controls of the game, choose how their character will grow, and fight their first boss. The player is guided through the Awakening by an inaudible, unseen "voice" whose identity is uncertain. 

To reinforce the dreamlike setting of the Awakening, Sora and Roxas both reach it by falling into an ocean. However, instead of drowning they find that they can breathe, and land upon an ocean floor which erupts into doves, revealing the Awakening. In Roxas's case, this first Awakening is merely a memory of Sora's, and he enters his true awakening three days later. The Awakening is composed of several enormous, illuminated stained-glass pillars known as "Stations", rising from unseen depths. Each Station has a unique shade, depicts a specific set of characters, and has its own specific part in the tutorial. In the first game, there is an additional area of the Awakening based on the Destiny Islands. The background music, "Dive Into the Heart (Destati)", initially begins quietly and hard to hear, but grows in intensity as Sora or Roxas approach the final pillar, where they fight a manifestation of the darkness within them. At the end of their battle with the creature the Awakening is consumed by the darkness emanating from it, dragging Sora or Roxas with it, but they either wake up or are rescued.

Kingdom Hearts Birth by Sleep
Ventus's pillar is similar to Roxas's in Kingdom Hearts II; it features Ventus instead of Sora,and displays the landscape of Keyblade Graveyard on the bottom, with five copies of the good luck charms made by Aqua on the top. Black versions of the symbol found on Master Eraqus, Terra, Aqua and Ventus's clothing encircle the pillar in green circles.

Kingdom Hearts
Each of the Stations depict the Princesses of Heart and characters related to them. The four that are clearly shown are asleep, perhaps signifying their comatose state in Hollow Bastion.
 * The first pillar, shaded green, depicts Snow White, the Seven Dwarfs, the forest animals, and The Queen in her hag form. While Snow White is asleep, all the other characters are depicted as awake.
 * The second pillar, shaded violet, depicts a sleeping Cinderella.
 * The third pillar, shaded pink, depicts three princess silhouettes, representing the three free Princesses of Heart, Alice, Jasmine, and Kairi.
 * The fourth pillar, shaded red, depicts a sleeping Aurora and silhouettes of Flora, Fauna, and Merryweather.
 * The final pillar, shaded yellow, depicts Belle, The Beast, and his servants. While Belle and the servants are asleep, The Beast is depicted as awake, signifying his freedom.

On the first pillar, Sora is taught to walk, then is shown three weapons, the Dream Sword, the Dream Shield, and the Dream Rod. He is directed to choose one and discard another, and his choice determines his specialty, initial stats, and ability growth throughout the game. After choosing, the Station shatters and Sora falls to the second pillar, where he first encounters Shadows, and is taught about battling and its rewards. The Station then sinks into darkness, and Sora awakens on the third pillar, where he is taught how to examine and interact with objects. By opening a treasure chest, moving a crate, and destroying a barrel, he materializes a door that leads to a dream version of Destiny Islands, where Sora meets facsimiles of his friends, Tidus, Wakka, and Selphie, which ask him enigmatic questions that determine how he gains experience throughout the game.

On the fourth pillar, Sora is again attacked by Shadows, but after defeating them the first Save Point is revealed, as well as floating steps to the final pillar. However, the steps vanish as he climbs them, trapping him on the fifth pillar. As he approaches a strange light, his shadow grows and becomes his Darkside; though he fights it, its darkness swallows him, and he reawakens on the Destiny Islands.

The door reappears, this time on Destiny Islands, when the island is attacked by the Heartless, revealing that it is in fact the world's Keyhole. It reappears a final time with the false Destiny Islands at the Final Rest within End of the World, but this time contains the final confrontation with Ansem, and Kingdom Hearts itself.

Kingdom Hearts II
On the 3rd day of his story, Roxas encounters a Dusk in the Sandlot and is sucked into his Awakening. Like Sora's, Roxas's Awakening consists of five stained glass pillars, but each of these depict the same image of Sora, Riku, Kairi, Donald Duck, and Goofy at the Destiny Islands, and the pillars are divided between only three Stations. While Sora is asleep, the others are all depicted as awake, even though Donald Duck and Goofy are resting in Twilight Town with Sora. Roxas awakens on the Station of Serenity, where he chooses between the Dream Sword, Dream Shield, and Dream Rod. While this choice determines the order in which he learns abilities as before, the weapon he chooses is immediately transformed into the Kingdom Key (This is different than in the original Kingdom Hearts Awakening as he does not have to give anything up in exchange for his weapon). He goes through a door to the Station of Calling, consisting of three pillars shaded blue, green, and red and connected by floating stairs. Here, he encounters Dusk and learns about Reaction Commands and treasures. Going through another door, he enters the Station of Awakening, and finds an apparition of himself in his Black Cloak which soon becomes his Twilight Thorn. Similar to Sora's Awakening, after defeating the Twilight Thorn, Roxas and his Awakening are consumed by darkness, but unlike Sora, Roxas is rescued by Naminé at the last minute.

Much later, as Sora approaches Memory's Skyscraper, Roxas reawakens inside him and takes him to the Station of Awakening, where they battle and come to peace with each other. This arena is replicated in the Garden of Assemblage for the fight against Roxas's Replica Data.

Kingdom Hearts coded
Data Sora wakes up in a digital version of the Awakening similar to Kingdom Hearts. He meets a black-coated figure, who then disappears. Unlike the real Sora's, Data Sora does not fight the Darkside here.

Trivia

 * In Roxas's Awakening Sora's eyes are closed, showing that he is asleep, while Kairi and Riku's are open, showing that they are awake; strangely, Donald and Goofy's eyes are still open, even though they are asleep as well.