Aww yea. Lookit that. What cute, innocent little kids full of hope and idealism and love and justice. Don't you just wanna run them through a death funnel adventure or a meatgrinder dungeon, or just generally destroy everything that made them beautiful inside?
Of course you do. That's why we DM.
So I've written multiple Mahou Shoujo/Magical Girl classes, for various subgenres of the archetype. I'm absolutely obsessed with the genre, having grown up on it and having had all my major childhood heroes that inspired me come from works generally based on it. I like Noblebright. I love Romantic Fantasy. I adore the power of Love overcoming everything in the end.
This first class is not that.
If you could have any wish, what would you trade your soul for?
That question is the birth of every Puer and Puella Magi, the slave-children of magic. Everyone who can call themselves such was approached in their adolescence, typically between 9 and 16 years old, though there are exceptions to every rule. 30-year olds with children of their own have heard the call, rarely. Gender is technically no object, but a majority of girls are approached.
The Incubator, normally calling itself "Kyubey" is a Messenger of Magic from another world. Although magic is born of emotional power, it has no emotions or true magic of its own, being a servitor of a higher Lawful force. Incubators exist only to form Contracts with potential Puella Magi, and in so doing grant them a wish in exchange for the duty and power of what they are to become.
The more hopeful and idealistic, the better. A good-hearted, innocent Puella Magi can shine like the noblest of Paladins. For hope is the greatest spice to create despair...
Soul Gems, being magical items, have some resistance to shattering despite their glass-like qualities. They're about as hard as a ruby.
Create Magic Weapon: You peer into the Soul Gem and find a vicious weapon ready to take shape once you pull it out of your Gem. If they are lost or broken, they can be recreated. At 3rd level, the weapon possesses a +1 bonus.
None save the Puella Magi can use the weapon, and they have proficiency in whatever weapon they choose it to be, which cannot be changed later.
Bodily Improvement: You are able to veer into the Soul Gem's true limits and in doing so, realize that you can push your own limits.
For the price of 1 Corruption Points, you can heal yourself for 1d8 + your Constitution modifier of hit points. This applies even if your HP is at -10 (or 0 HP, or whenever people become corpses in your game).
For the price of 2 Corruption Points, you can cure yourself of a disease, though it will be 4 Corruption Points if it's brought on by a spell or otherwise supernatural. Alternatively, you can temporarily increase one of your Ability Scores (excluding CHA) by 2 for 1 hour. This can stack with itself.
For the price of 4 Corruption Points, you can regenerate lost limbs or any other severe handicaps, as well as give yourself rerolls on saving throws.
For the price of 9 Corruption Points, you can recreate a destroyed body. This will take a few minutes, because it's basically either recreating a body from scratch or rotting a corpse in reverse.
Name Level: At Name Level, you become known as a Prince or Princess of Magic, and have demonstrated enough competence and luck to have lived this long that Kyubey has made you less of a target for harassment, and now uses you as a poster child for recruitment. Pick a settlement, and you become the de facto leader of it in the Puella Magi world, claiming it as your exclusive hunting grounds. You attract 1d6 novice Puella Magi who look up to you and see you as their senpai. Due to the lack of reliable information-durability due to the Puella Magi life being full of secrets and isolation, rumors about how you treat your new teammates don't spread very far. Indeed, if one of them dies and they didn't have a very public presence, most won't realize they had ever gotten involved with you at all.
Corruption Points: Throughout the class, you noticed how the words "Corruption Points" have been uttered. This is something that you gain when you make the Contract. You start with 0 Corruption Points in a pool known as a Corruption Pool, but with each use of your abilities as listed, you gain Corruption Points. Your Corruption Pool can only take so much Corruption before it exceeds its limit. Its Limit starts at 10 Corruption Points, with an additional 2 added for each level above 1. You also add your Charisma modifier to your Corruption Pool at every level including first.
The Corruption of a Soul Gem represents grief and despair, so aside from magic use, intense negative emotion can also cause it. If anything prompts an intense episode, such as a roleplaying scene, losing a dear loved one, or anything like a Save Vs. Madness or a Crushing Despair spell or whatever, have them make a saving throw (whichever correlates closest to a Wisdom/mental stability-based save), and if they fail, they gain a Corruption Point.
For every 10% of your Corruption Pool that's filled up, you take a -1 penalty to saves against future despair-induced Corruption saves, to reflect your darkening mood and imminent grief-spiral as the hope within you flickers and dies.
If your Corruption Pool reaches its limit, your Soul Gem bursts and becomes a Grief Seed, as you turn into the very Youma you've been hunting. Oh my god, what a twist! If your players are somehow unfamiliar with the reference material, hide this part, it'll be fucking awesome.
The only way to lower Corruption is to use Grief Seeds. Every defeated Youma drops a single Grief Seed, which refreshes Corruption Points as if they had a Cleansing Pool equal to the Corruption Pool of a Puella Magi of its HD, minus any Charisma bonuses . Excess points are not lost when a Grief Seed is used, so a Puella Magi can hold on to a half-spent Seed. A fully spent Seed can be used to cleanse a Soul Gem, but going over the Cleansing Limit causes the Grief Seed to hatch into a Youma, which will have 'leveled up' from the extra Corruption.