Written in Blood

Warlocks. Foul creatures, born of human stock and bound to some higher being for power. Each is defined first by patron and second by pact; the entity to which they owe their power, and the gifts they receive.

You and your compatriots are some such wretched beings.

Michael Maier

At each level, warlocks gain a Magic Die, a d6 representative of some shred of power or minor favour owed to you. Some pact abilities give you the opportunity to roll them. [dice] is the amount of MD you commit, [sum] is the total of all results. MD are expended on a 4-6; dice that roll lower return to your pool to be rolled anew. Whenever you roll doubles, consult your patron's Mishap table; the corresponding result occurs (double sixes result in the sixth entry, double threes the third, &c.).

All warlocks are fated to meet their demise. Whenever a Mishap occurs, note it. Whenever you make the third strike, a Doom is invoked—some greater catastrophe. As with Mishaps, the exact consequences depend on your choice of patron. Few, if any, survive their third Doom.

Patron: The Fiend

Hob. Old Scratch. The Devil. The Fiend bears a thousand names in as many different tongues, but all refer to the same being—the lord themself of the lower planes. They seek to bring out the worst in humanity; violent, impulsive, brutal. All in their thrall embody these virtues, in one way or another.

Michal Kváč

Perk: Can summon a flame as that of a candle with a snap of the fingers.
Drawback: Sacred objects burn to touch.

The boon of your pact—book, sword, or servant—looks as an object of the Hells should. Earthen or crimson tones, a charred scent, wicked spikes, infernal inscriptions.

To forge a pact with the Fiend—whether you sought it out, or it was brought to you—one must take a dip in the fires of the underworld. Few pilgrims make it through unscathed. The mark you bear takes the form of...
1. A marred face.
2. Eyes turned an unnatural shade.
3. A pair of horns jutting from your forehead.
4. An arm-length tail.
5. A shadow twice the length it should be.
6. Breath that reeks of brimstone.
7. A network of scars across your forearms.
8. Bodily fluids (blood, sweat, tears, &c) the deepest black.
9. Nails as sharp as claws.
10. An extra set of canine teeth, jutting out of your mouth.

Mishaps:
1. 1d6 damage to you.
2. MD only return to your pool on a 1-2 for 24 hours.
3. You lose all benefits of your boons for 1d6 rounds.
4. Random hellish mutation for 1d6 rounds. Save with advantage or permanent.
5. Fire spurts out from you. 1d6 damage to you and everything within 2 metres.
6. An imp pops into existence. They seek to cause maximum mischief for those around them until killed and can summon minor illusions—cause flames to flicker or dim, slam open doors shut, call forth harmless tremors, &c.

Dooms:
1. Die for a day. Your soul is taken temporarily to the Underworld, for fiends to do with as they please.
2. As 1, but for the person you care about most in the world.
3. As 1, but for eternity.

Pact: The Blade

You have chosen a path of violence. Your boon takes the form of a weapon of your choice—dagger, sword, axe, &c. You are as skilled with it as any other combatant, and are bound to the blade as much as it is to you.

Gilles Ketting

A: Boon of the Blade: You have taken a weapon of your choice as the object of your pact. It counts as magical, its appearance influenced by your choice of patron. It vanishes into nothing when separated from your or at a moment's thought, and you can summon it from the aether with 10 heartbeats of concentration (a single action in combat).

A: Bloodthirsty Edge: Whenever you make a successful attack against an enemy, you may roll any amount of MD. If you choose to do so, the attack inflicts an extra [dice] damage. Upgrades to [sum] at C.

B: Deft Hands: You've grown more comfortable in your weapon. You never fail to do things with it other than attack—disarming enemies, tripping foes, striking them with the flat of the blade, &c.

C: Indomitable: Your body has improved, infused further with magic. You can roll MD when making saves to add an additional [sum] to your result; whenever you step out of the fight for a moment to take a breather, you may regain [dice] health rather than the norm. 

D: Flurry of Steel: You have been gifted a preternatural speed in combat. You can make two attacks with your pact weapon on each of your turns without penalty. 

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The beginnings of a potential hack, of sorts, replacing the 'race'/class dichotomy with patron and pact. Hopefully makes a degree of sense. Feedback of all forms appreciated—as are riffs on the same concept.