You have a magical being or races of beings in mind, here are some questions you should ask yourself:
How will they collect it?
Is the power in their bodies? Do they have to collect it from some outside source? Or, is it a combination of the two? You don't need to fully flesh out the power source now. You can, but you don't have to. I nailed down the source much later in the process and will talk about that in the next post of this series.
Once collected, how do they use it?
As I said in the previous post, my character draw power from Darkness. The power lives inside them. They have to train it. Once trained, they tell the power what they want then, the power works with the body to make it so. Do your characters have to chant to use it? Do they need to connect to something like the elements?
How does it live in the body?
Not just live, but sit. Your characters are pulling power into the bodies to push it out or the power is already in their bodies and they need to push it out. Either way, at some point, it sits in the body. How does this look? Where is it? Does it physically live in the body or sit in some untouchable place like the soul? To understand how the power lives in my characters' bodies, I drew this out. It helped a lot.
Because of the power, my characters live for centuries. They heal quickly and age slowly.
What are the consequences, weaknesses?
You've probably heard this before, "every character needs a flaw." Every power needs a weakness. It's boring reading about all powerful characters. From reading about different powers, it usually has some affect on the body. I'm not going to tell you the Sciell's weakness because it'll give away the story ;) But, it does have to do with the body.
Do the characters become exhausted after prolong use? Do they pass out, become ill? If they absorb too much, do they risk all their cells breaking down? Since I already told you about the beings who become shadows- just so you know, they are no longer called Shadows. Because they give their bodies to Darkness, they have this violent animal side. They have to work to suppress that side of themselves.
I'm hoping the answer is no because that could make for a really boring story if everyone uses the power the same way. Figuring this out will take some time. It will develop as you're writing your story. If you're like me, you don't outline, you make it up as you go along. At some point, you'll need to list out all your characters and their powers. I'd share mine with you, but it'll gave away the second book in the trilogy ;)
How are these differences determined?
For the Sciell, the way they mold their power can be hereditary, but not always. It's not exact. If the use of the power is unique, then the parents passes it on to the child. More often, their bodies determine how they use their power, which means the way two characters mold the power to do something will be different.
What are the characters' weaknesses when using the power?
This can go along with consequence and how each character uses the power, but I wanted to make this separate. In The Merging Worlds Series, using the power for everyone has a weakness, but each individual also has a weakness. For instance, a character who is adept at using the power for long range attacks may be terrible at using it in close range. A character may have a awesome ability, but it takes them longer to mold it.
Does the use of power vary between races of beings?
In the fantasy stories I've come across, each race of beings uses the power differently. You can have a world where the different races are distinguished by how they use the power. In my story, certain races are more adept at using the power to do certain things. And, certain characters are better at molding the power to do some things. It doesn't have to vary though. You could keep the differences at the character level and have the races be distinguished by something else like religion or location.
If you have any questions, let me know!