Okay, so I had this idea randomly today.

There's this witch that offers young couples anything their heart desires, in exchange for their firstborn child. The witch is this grungy-looking ugly old hag, so many people in the kingdom, and even neighboring kingdoms are afraid of her. 

In reality, she's a very nice woman, the reason why she takes children away is so they don't grow up with irresponsible parents that make deals with a witch in exchange for their child. They all see her as their mother, usually calling her "mama" or some other variation of that. She also teaches the kids different magic: spells, potions, incantations, you name it. The witch hopes she can teach the kids well enough to eventually take over her role when she passes.

Now I have two different ideas for this book:

A) The witch has gone missing/has been murdered (to be decided later) and the oldest of her adopted children go out to find/avenge her. The main character would be the oldest child.

So in that version, there'd be a group of maybe 5 or 6 kids, 4 (or 5) of them are between the ages of 14 and 17, the last one is about 5ish 6 years old. The reason why they take the youngest along is that they're the best in healing magic, plus they begged the older kids.

They go on an entire adventure trying to find their mother and run into some royals along the way, it's a prince/princess. The oldest of the group (the main character) wants no part of including the royal, because they believe that they can protect the group no matter what happens, spoiler, they can't do everything by themselves. Eventually, the main character falls in love with said royal, because yes. 

Other side things happen during the book, but I don't want to give much away.

B) It takes place when the witch is younger before she decides to make deals with irresponsible couples. She on the run with the law since magic is considered extremely dangerous and illegal, the punishment being public hanging or burning. 

This version would basically be a lead-up point to where she is in the version above, kind of like a backstory for the witch. She meets a young man, around the same age as her, while she's running. They quickly fall in love and start living together, this man means a lot to the witch, since she decided to settle down and risk being caught. 

It would be later revealed that the man betrays her, he sells her out to officials so she can be burned. But only after they get intimate together because he's an asshole. She's actually about to tell him about being a few months pregnant when she finds out that he's sold her out. On her way to the wooden pole/pile where she will be burned, she curses him. That as long as they live, he will never find solace in another person, whether it be romantically, sexually, or platonically, there will never be another person that loves him.

The witch escapes her public burning ceremony, barely, and runs into the woods. After a mile of running, she stops and collapses of exhaustion, this is when she realizes that she was badly hurt. Healing takes a lot of stress on the person who does it, it doesn't damage them physically, but it makes them extremely tired, use too much energy to heal and the castor dies. She tries to heal the wounds that were caused by her captors but is exhausted. She has a miscarriage.

After that, it's basically her trying to rebuild herself and get over her former lover. She realizes just how many selfish people there are in the world and vow to help as many innocents as she can, which are mostly children and babes.

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As I finish writing this, I realize that making option B a prequel for option A would make more sense, even tho I kinda wanna write the witch being a complete badass xD

Which one should I do?

Write A then, later on, B as a prequel

or

Write B then, later on, A as a sequel