Within you, something grand has awakened. A power so imbedded within you, it is (in its own way) a deeper aspect inside of you. This power is not mystical, technological, psionic, or even spiritual, it simply exist within you. No form of anti-power can stop its actions or deny you entry into its depths, not even the gods on high or beasts beyond the abyss. The madness they induce or even the theft of one's own soul will not stop you from accessing the world. This Inner World that you have now shapes itself as a representation of your own existence; everything you remember to everything you've forgotten, to every success and every failure, and to your greatest pleasures to your worst fears. This place will take these aspects and harden them into a form that you can interact with them so much so that it is indistinguishable from reality. What does it do with such experiences? Well it shapes them into challenges, trials, quests, puzzles, enemies all which guard rewards that represent the crystalized experiences that they have gone through, these items are also known as Birthrights. Let's say that one has a phobia of spiders, one could simply kill the beast and obtain it as a shade/minion to have in battle or one could appease the beast and make friends with it obtaining a Birthright that makes them friendlier to arachnids and get them to help you along the way. These challenges can be accomplished in multiple ways since people and even your are complex beings, there is no wrong or right way to complete them, but be warned the Birthrights power and strength will sync to how the challenge was completed, so the easy way might make the Birthright weaker than it should be. But be warned full blown cheating or evading the challenge all together will result in consequences and no prize
The Birthrights are a wonder in their own right, they can be just about anything from the usual weapons and armor, but even more interesting are the potential esoteric objects that can be obtained throughout the Inner World. The can have mystical or spiritual properties or even be some form of advanced tech that is to unique to copy and only works for you. Maybe even pools of energy that can restore your vitality and enhance the abilities granted by the World itself. Now items granted will have an overall theme that is personal to whoever has the inner world, maybe the items have specific color scheme, or the majority of the Birthrights are jewelry that summons other objects instead of weapons and armor directly.
Now there are two ways to access this inner world; the first is the easiest through simply sleeping. When one is able to achieve peaceful rest, they can access their inner world with ease but it lacks the inherited awareness of the second option and if one is awakened at any point will throw one out of the world harshly. The second and best way is the act of meditation; by clearing the mind and making a mental simulacrum of oneself, you will be partly conscious of what goes on on the outside world so that if something occurs you can be prepared. Also time works a bit differently in there, several hours in the Inner World will be just 30 minutes in the outside world instead.
Finally, the most important part of the Inner World is not a constant and fixed location, just as people grow so can it. While personality-esque challenges take a while to generate, by having other powers the Inner world will create sub-worlds for them as well. Each with their own unique challenges and items to make them stronger, or even secrets to add versatility to them can be obtained. Usually powers of similar themes and/or sources will have their sub-worlds mixed so they can generate combined objects but if the power is distinct enough then it will gets it own area.
Remember this power is not only meant to make you strong but to also be used as a form of reflection into one's very being. To simply ask: "Who are you going to be?"