Dark
Where am I?
Alone
Hello? Anyone?
Empty
Who...am I?

That is most likely the last thoughts of a heartless before they are controlled. Their minds are to be lost.

Gone.
Forever.

It's something you'd never expect to happen. You live a life. Not a busy or exciting one. But a calm and easy going life with a share of troubles. Then you die. Most people expect that their hearts were strong enough to go a step further. However, it's not. In fact, most people's hearts are not. And after death, they're heartless. No hearts.
Missing.
Forever.

Everything in your mind. Everything in your soul. Everything in your heart. Never to be seen again.

Once a heartless, you lose everything. Right before you're under the Darkness's power, your life. Pathetic life. Is a flash back. By the end, it shows one last flash. Your most happiest moment. You chase after it, but then it hits.
Nothing.

You are now nothing. The Darkness shrouds you. Telling you everything is fine. Nothing will hurt you.
Lies.

The silence. The last of what you had is gone. You are darkness itself. It controls you. Keeps you going. You're dependent on it.

You do what you're told. You're reckless. Think you're invincible. Then it happens.
You die.
Once.
Again.

But this time it's for good. Everything you did as a controlled monster flashes in your refreshed mind.
Regret.
Guilt.

Then it goes black.
Everything.
Forever.