I really like the magic book that Jim Raggi put out for Free RPG Day this last year. You know the title, which I shall simply abbreviate as VAM because sometimes I blog from a work computer.
All the spells in the book are named for death metal songs, and at the end of the book, Raggi provides a list of 120 and invites the readers to invent spells based on these names. I rolled a 1d120 using random.org and rolled up the title Bacterial Surgery. I offer you now a spell I have created for use with Lamentations of the Flame Princess, specifically the new magic system offered in VAM, though you can introduce this into any OSR game. Make it a 1st level spell available to magic-users.
Bacteria are agents of entropy. They infest, they infect. They wait patiently until we die and our immune systems can no longer hold them at bay, and they break us down into putrid corpses and then eventually into bare bones. They consume produce, taint water, and otherwise bring about rot and ruin.
This spell takes the nature of bacteria and inverts it, turning them into agents of creation and construction. The bacteria inside an individual will begin repairing injuries and purging diseases, seeking to make the body whole, or as near to whole, as possible.
As she casts the spell, the caster must insert a finger into an orifice on the subject's body where bacteria could be found. (The mouth, for instance) The bacteria then spring into action, causing the victim to fall into a feverish sleep while repairs are underway. The subject regains 5 hit points per round, and every three rounds is purged of a disease or poison. The bacteria prioritize, so if the victim is dying from poison, it will address that poison first. Even a subject who has be reduced to 0 hit points can be revived if the spell is cast within 1d4 rounds of reaching 0 hit points.
Once the subject is fully restored, they awaken and noisily vomit a green sludge. Said sludge is worth 10 gp worth of components for a magic-user's laboratory. A person who receives the benefit of this spell cannot be healed by cleric magic for a week afterward. A cleric who receives the benefit of this spell cannot cast any healing spells for a week as well.
This spell has no effect on elves or dwarves.
1- The bacteria continue to repair the victim even after all injuries are healed. They instead begin building tumors, growths, and excess flesh. The subject may make a saving throw vs. poison each round to stop the runaway growth. Each round the growth continues, the victim gains another 5 hit points beyond their maximum, but they also lose 1d4 points of Constitution and Charisma. All changes are permanent. If a victim reaches 0 Constitution, their body implodes. If the victim reaches 0 Charisma, they can no longer pass as remotely human.
2- Doppleganger- the green goo vomited forth by the subject will slowly grow and mutate, becoming a perfect physical replica of the character after 24 hours. The replica has all the memories, but generally the reverse personality of the subject. The doppleganger's only desire is to murder and replace the original character. The doppleganger is statistically identical to the character, but will be unable to use any cleric spells.
3- The bacteria instead eat the insides of the victim, inflicting 5 hit points per round until dead. The victim then leaks green goo out of all bodily orifices. The body left is a hollow shell that cannot be restored by a Raise Dead spell. The victim may make a saving throw against Poison each round to stop the process.
4- The bacteria hollows the victim out, as above, but then animates them. The subject is essentially a zombie with similar stats, but it isn't affected by anything that harms or repels undead, and it takes only a single point of damage from any weapon attack.
5- As above, but the bacteria-zombie leaves the brain intact and therefore can access the memories, personality, and skills of the original character. The bacteria zombie seeks only to spread it's bacteria, which it does by secreting saliva into an orifice or open wound on the body of another sentient being. The victim will go comatose in 1d6 days anad suffer the effect of #4 on this table.
6- As above, but the bacteria zombie can now transmit its infection via bite, and all people it turns suffer this result on the table.