Cancer Mage (Onion's version)

In the shadows within which you dwell, you discover a frightening magic power; or perhaps it discovers you... The majority of the poor fools inflicted with this evil, intelligent sickness do not follow this archetype by choice, and those who do are usually among the demented dregs of society. However it finds you, tumorous growths, pustules and blisters that sprout from your flesh grant you knowledge of abilities beyond what you would normally be capable of. You learn to use your new putrid skills to fell your enemies with infectious diseases and deadly poisons, which you can conjure from the decay that festers within your mind and body. Cancer mages enjoy the company of filth, and avoid sunlight and large crowds. More so than most rogues, they're vile recluses who choose allies very carefully.

(Me and my friends saw this in the 3.5e core book "Book of Vile Darkness" and thought it was really absurd, so I tried converting it into a playable subclass for 5e mostly for personal use. This is my first time uploading anything publicly on DDB hehe so pardon any misspellings or weird balance issues with it, I tried my best :,))

Venomous Sting

When you use your sneak attack feature, you can choose to change the type of damage done by your weapon into poison damage. If you do this, you can force the target to make a Constitution saving throw against your spell save DC. If it fails, the target is poisoned for an amount of hours equal to your proficiency bonus. You are also able to sense the direction to its location if it is within an amount of miles equal to your Charisma modifier (minimum of 1) away from you while it is poisoned in this way.

Cancerous Companion

The cancerous magic within you manifests as a second head somewhere on your body. It has an identity and personality of its own separate from the cancer mage, and it can communicate telepathically with its host. When needed, the growth shares your ability scores for saves, checks and so on. In addition, a number of times per long rest equal to your Charisma modifier (minimum of 1) you can use a reaction to shift your concentration on a spell over to your growth when a creature you can see hits you with an attack, halving the DC of the Constitution saving throw to maintain your concentration.

Filthy Friend

Instead of taking the cancerous companion feature, you can take the filthy friend feature. Once per long rest, you can spawn a small creature companion from one of the pustules on your body, taking the form of a slimy pigeon, cockroach or rat. In combat this creature takes its turn directly after yours, and you can command it to act however you desire. At 9th level, you can spawn 2 filthy friends when you use this feature. At 13th level, you can spawn 3 filthy friends when you use this feature.

The stat blocks for these creatures are as follows:

 

Perfect Host

You become a carrier of every disease you encounter, and are immune to the effects of disease aside from cosmetic ones, such as pockmarks, pustules, warts etc. You gain resistance to poison damage. If any spell that would remove disease is cast on you, you take 1d8 force damage per level of the spell slot it was cast with.

Meat Shield

As a bonus action, you can form a shield of maggots around your body to guard you from harm. When you use this feature, you gain temporary hit points equal to your rogue level + your Charisma modifier. In addition, your armor class increases by 2 while you still have the temporary hit points from this feature. When the temporary hit points from this feature are reduced to 0, all creatures within 5 ft. of you take acid damage equal to your Charisma modifier.

Evolved Infection

Your weapon and spell attacks ignore resistance to necrotic, acid and poison damage. Creatures that are immune to necrotic, acid and poison damage will remain unaffected. In addition, when you poison a creature it cannot be cured by nonmagical means. Creatures poisoned by you can only be healed via a casting of lesser restoration at 6th level or higher.

Disease Form

You can use an action to transform yourself (and everything you're carrying) into an airborne disease.

While in this form:

- You are completely intangible and are invisible to standard forms of observation, even blindsight.

- You cannot move unless you are being carried by the wind.

- You can use an action to attempt to infect a creature within 100 ft. of you. Your victim must make a Constitution saving throw against your spell save DC, it automatically succeeds if it is under the effects of the protection from poison spell. If it does succeed it takes 3d6 acid damage and it becomes immune to your disease form for 24 hours. If it fails, you enter into its body as a disease indistinguishable from a common cold for 24 hours, at the end of which your host will take 1d4 Constitution damage and make the saving throw again, granting you another 24 hours in its body on a failed save. This process is repeated until you are either ejected or your host dies.

- If you are inside of a host you can use your action to take total and precise control of its body. Until the end of your next turn, your host creature takes only the actions you choose, and doesn’t do anything that you don’t allow it to do. During this time you can also cause your host creature to use a reaction, but this requires you to use your own reaction as well.

- If you are inside of a host, you can use an action to attempt to infect an additional creature that moves within 10 ft. of your host. 

- You can be forcibly removed from a host if lesser restoration is cast on it at 6th level or higher. If this happens, you are shunted from that host's body and take 1d8 force damage for each of your hit dice. Your victim will begin to recover normally.

- If your host creature is reduced to 0 hit points while you still occupy its body, it is instantly killed and your disease form is dispersed above its corpse. You can then once again use an action to attempt to infect another creature within 100 ft. of you.

- If you are reduced to 0 hit points, you will be ejected from any hosts you occupy, revert back to your standard form and fall unconscious.