Declassifying Ouroboros

The idea of normal D&D classes could easily be applied to Ouroboros - just run LotFP or B/X or whatever your system of choice is, steal some tables from Last Gasp Grimoire (plug), and you're probably good to go!

That being said, I had a more esoteric idea for how this could work. Instead of picking a class and sinking levels into it with XP won through combat and plunder, just join a society and move up through the ranks to unlock new powers. It's a little bit based on the "Special Abilities & Permissions" section of the Blades in the Dark rulebook.

So here are the base classes (cleric, fighter, thief, wizard) replaced by the society you could join to gain their abilities.

The Cult of Ishtar

Initiate

Joining: Liberate someone from an unhappy marriage.
Benefits: You get 1 favor die (d6), which is depleted for the day on a 1-3. If it is not depleted, next time it depletes on a 1-4, and so on. Rolling a depleted die still works, but you must roll on the Divine Punishment table. When you kiss a wound, you may roll a favor die to heal the damage and close the wound.
Restriction: You must renounce your gender and wear only the pearls and robes of the courtesan. If you do, you will be slathered in honey, filled with wine, and excommunicated.

Adept

Joining: Shelter one who has been done violence by their lover.
Benefits: You get a 2nd favor die. When you kindly ask someone to do something, you may roll a favor die to magically compel them to do it. This compulsion lasts until the task is complete.
Restriction: You must never turn down a request to aid true loves course. If you do, you will be forced to re-enter the cult as an initiate.

Master

Joining: Smite a powerful person who has abused their lovers.
Benefits: You get a 3rd favor die. When you or an ally is attacked, before the roll is made, you may roll a favor die to force the attack to miss. Alternately, when an ally makes an attack, before the roll is made, you may roll a favor die to force their attack to hit or to add it to their damage.
Restriction: You must never wield weapons. If you do, Ishtar's flaming sword will pierce your heart in the dead of some soon-to-come night and your soul will never be taken to her boudoir.

*Divine Punishment Table*

Effects, unless otherwise stated, last d6 hours. Rolling a 6 means it's d6 days, and then weeks, then months, and so on.

1. The target suffers stigmata and begins to exsanguinate, suffering d4 damage every round for d4 rounds. The blood pouring from their wounds is of the sweetest votive wine, and would make a great offering to Ishtar.
2. The target's eyes swell and slit like snakes', all seems painfully bright to them and they feel the urge to slither away into somewhere dark and damp. They take -2 to hit / AC in direct light, but they can see in the dark.
3. A crown of spiraling goat horns push from the target's head and their breasts grow massive (if they didn't have breasts, they do now!). Their constant lactations, if collected into a wine bottle, heal d4 HP.
4. The target grows delicate, graceful wings like that of a moth. They are vestigial, but beautiful in an ethereal sort of way.
5. in patches, you can see tiny shapes squirming just beneath the surface. When attacked from behind they take double damage, and the tiny bastard spawn of Ishtar will spill forth.
6. Their eyes, nose, mouth, and other orifices constantly ooze honey of richest amber. It attracts clouds of stinging, biting insects that fortunately die shortly after their first taste of Ishtar's sweet nectar.
7. Twisting horns and jagged fangs burst from the target's joints, dealing d6 damage and forcing them to move at half speed, lest they take another d6 damage.
8. White lilies sprout wherever the target's blood lands, if ground and ingested they grant +2 to physical checks for an hour, but after a week the madness sets in, and after another the change comes over them.
9. Ishtar boils your blood for a few moments, causing you to vomit up caustic sanguine blobs and dealing you d6 damage.
10. Angelic organs and prehensile things burst from the target's mouth, fronding the air, the only noises they can make are of the most beautiful music, even their screams for pain are choral and harmonious.
11. The target's saliva transmutes into a powerful aphrodisiac which they're at danger of swallowing to the point of overdose.
12. The target's tongue is replaced by a black adder, malicious and intelligent, though wishing no harm upon the target themselves. It will depart back to Ishtar once allowed to lethally envenom someone.
13. Ishtar finds the target's form wanting and reshapes it. Re-roll all physical stats. The target finds themselves now androgynous and beautiful.
14. The target's eyes turn into valuable exotic red gems, their skin to soft and pliant marble. They can still see and feel just fine, and don't get any fancy powers or a boosted AC.
15. The target's skin becomes glistening and night-black, their hair falls out and their nails pop off, their teeth sharpen into small fangs, they take on an overall Giegeresque appearance.
16. The target falls into writhing ecstasy, unable to act or speak in any coherent fashion.
17. Ishtar's glory manifests in the target, forcing everyone who can see within 30 feet to save or be overcome by ecstatic melancholy, falling into an orgiastic frenzy as they are overcome by hallucinations of beckoning genderless figures.
18. The target begins fading from reality. Every day they must save or advance down this track: Real --> Insubstantial --> Invisible --> Gone. The only way to end the march towards vanishing is to fall in love.
19. Ishtar finds your devotion wanting. Prove yourself forever beyond all question, right now, or have your pitiful body returned to the primordial squirming flesh-mass it was always meant to be.
20. The next time they sleep, the target must save or go to Ishtar's boudoir in their dreams and never return.

A priestess of the Lover of Lovers

Blackheart's Fencing Club

Initiate

Joining: Defeat an initiate of the club in a formal duel, proctored by an adept.

Benefits: Choose a signature sword and design a trick for it that you can perform instead of attacking. Also, you get +1 to hit with your signature sword.

Restriction: You must never refuse a duel, and must duel at least once a week. If you fail to uphold either of these commitments, you will be banned from entering the club.

Adept

Joining: Defeat an adept of the club in a formal duel, proctored by a master.

Benefits: Once per combat, you may parry a blow that would hit you, ignoring all damage. Design a new trick, and get another +1 to hit with your signature sword.

Restriction: You must only fight with your signature sword. If you use another weapon, you will be kicked out of the club, forced to win your way back in.

Master

Joining: Defeat a master of the club in a formal duel, proctored by a gathering of the other masters.

Benefits: Attacks with your signature sword ignore AC provided by armor. Design a final trick, and what the hell, get +2 to hit with your signature sword.

Restriction: You must never wear armor. If you do, the other masters will hunt you down and kill you without honor.

A club member

The Hand

Initiate

Joining: Steal a treasure worth 200 silver and gift it to an adept.

Power: Once per day, when you are restrained (physically or socially) and you could feasibly escape, you do. You get +1 to hit from hiding.

Restriction: You must fence everything with the Hand, and hand over half the take to the Hand. If fail to uphold this commitment, you'll be kicked out of the Hand and blacklisted by every fence in the city.

Adept

Joining: Steal a treasure worth 1000 silver and gift it to a master.

Power: When an attack misses you, you may redirect the attack to another target, forcing them to roll a new attack. You get +1 to hit from hiding.

Restriction: You must never pay for anything. If you do, the other adepts will attempt to steal everything you own and hold dear.

Master

Joining: Steal a treasure from the Queen's palace and gift it to the Hand's coffers.

Power: Once per day, you may leave a scene and later declare yourself to have been disguised in the background all along. You get +1 to hit from hiding.

Restriction: You must never honor a deal. If you do, the other masters will attempt to steal your life.

Light fingers

The Mantic School

Initiate

Joining: Bring an artifact or spell grimoire recovered from another Ouroboros to an adept for verification.

Power: You get 1 mantic die (d6), which you roll to cast a spell. On 1-3 the spell still casts but the die is depleted for the day. If it is not depleted, next time it depletes on 1-4, and so on. You may push a depleted die to cast the spell anyways, but you must roll on the calamity table. You are taught 3 random spells by your new adept-teacher.

Restriction: You must spend every night studying your spells. If you fail to do this, you won't be able to cast the next day.

Adept

Joining: Bring back an unknown creature from another Ouroboros for study by the masters.

Power: You get a 2nd mantic die. You are taught 3 more random spells by your new master-teacher.

Restriction: You must take on an apprentice selected for you by the masters and teach them 3 spells. If you neglect your duties or death befalls your pupil, you will be forced to re-join the school as an initiate.

Master

Joining: Bring lore about the Mantic Demon and how one might defeat it to the masters.

Power: You get a 3rd mantic die. You develop your own signature spell, which you can cast once per day without rolling a mantic die.

Restriction: You must never push a depleted mantic die. If you do, the old Mantic Demon will come for your soul in the night, it can be fled from or driven away but never truly defeated until it claims you.

*Calamity Table*

1. Arcane flames wreathe you, scorching your flesh and dealing you d6 damage. You will heal somewhat, but will always bear the marks of witchfire.
2. You age 5 years an hour. At age 100, you die and are reborn as an infant out of your corpse. The cycle begins again.
3. You gain a random insanity from the Gauche Madness table (a post yet to come).
4. The spell takes form, it holds nothing but hate for you and is possessed of a terrible intelligence, defeat it if you'd ever like to cast it again.
5. Your face becomes a spiral of whirling shattered crystal, you are unable to speak  but may still cast spells.
6. Strange fungus blooms from your back and sporulates aggressively, affecting everyone within 30 feet. This happens every time you cast this spell now.
7. A tiny version of the Mantic Demon in one of its many guises claws its way up your throat, dealing d6 damage on the way out before scampering off to scheme and sow mischief.
8. You gain a random insanity from the Urbane Fervors table (a post yet to come).
9. Caustic green mist pours from your mouth with the words of the spell, choking you and everyone within 30 feet who fails a save into painful unconsciousness. While knocked out, they dream dark and prophetic dreams.
10. You gain a random insanity from the Eldritch Lunacy table (a post yet to come).
11. Your skin begins to shine reflectively, your joints crackle when you move. You have become somewhat crystalline, with no obvious downside.
12. You wretch violently, borne to the ground by the force of the heaving, what comes forth are wet tendrils with no apparent other end, curling and twisting in the air.
13. You swell and swell, witchfire pouring from your mouth and eyes and nose, dealing 2d6 damage to everyone within 30 feet as you explode, before reconstituting all wrong (re-roll physical stats with 2d6).
14. You gain a random insanity from the Gauche Madness table.
15. You gain a random insanity from the Urbane Fervors table.
16. You gain a random insanity from the Eldritch Lunacy table.
17. Everyone within 30 feet, including you, suffers a spontaneous infestation of tiny crawling things just beneath their skin. This happens every time you cast the spell now.
18. Your blood is slowly and insidiously replaced by infectious worms. They do no harm to you, their glorious host, but any time your blood contacts someone else's skin they will be rapidly devoured from within and turned into a parasitic monstrosity.
19. You are suddenly encysted in a cocoon of silver and crystal, lapsing into a delirious and delightful slumber, unlike any since you were a child. If no one penetrates the chrysalis within the hour, the thing they find inside will no longer be you.
20. The Mantic Demon comes for you in the night.

Master of the Mantic School