This Realm isn't going to be easy.
The only way you can appear within the dark realm is as followed.
1. If your character/character's friend heart turned dark, when they are defeated/near death, a portal will appear within below them which will take their body to the realm of darkness. If it's your character's friend, your character could take their place to save them which'll mean your character's friend will live but your character will have a harder chance to escape.
2. If Kingdom Hearts is opened, you'll need two characters to close the door. One closing from the outside while One closes it from the inside.
3. If your character has true ties to the darkness, either uses the darkness from the Dark Realm or is an apprentice of a Master of Darkness, they can make a portal to the dark realm but can only send someone in one at a time, they must find a way out of the Dark Realm through a different way.
This Realm will test your heart.
If your character is strong within their heart, powerful with the light or darkness, they might survive it's depths. This is not a guarantee. The only known survivor that escaped from the Dark Realm was Aqua and she didn't escape for many years.
Time moves slower in the Dark Realm
This realm seals everything, including time. Your character will age very slowly to the point where it may be only days for your character inside the realm but it could be many years in the outside realms.
You have been Warned.