Cancer Mage

A cancer mage is one of the worst and most disgusting "people" a person is most likely to meet. They are ravaged and littered with disease, filth, and decay. As a vile creature, the cancer mage is most likely to have dark goals and to worship gods who favor corruption and disease who spread disease simply for the sake of disease.

Disease Host

The cancer mage becomes a carrier of every disease he encounters, though he remains immune to most of their effects. However, the cancer mage takes 1d6 points of damage per spell level if someone casts Lesser Restoration spell, Greater Restoration spell, Heal spell, or Lay on Hands on the cancer mage he can make a CON save to try to resist the effect / Equal to the caster's spell save DC). Furthermore, if the cancer mage has a cancerous companion, he loses all abilities gained from the companion for 1d10 days after being the subject of Lesser Restoration spell, Greater Restoration spell, Heal spell, or Lay on Hands.

Cancerous Companion

This cancerous growth gains intelligence and a personality distinct from the cancer mage. The companion and the cancer mage can communicate telepathically. When needed, the cancerous companion uses the cancer mage's scores for saves, ability scores other than Intelligence, and so forth. The cancerous companion gains abilities as the cancer mage gains levels.

Contagion

The affected creature must make a CON save. On a successful save nothing happens to the creature. On failure the creature may take 3d8 Necrotic damage and have the Poisoned condition, the caster must also choose 1 ability upon using this feature, the afflicted creature has disadvantage on saving throws with the chosen ability (the affected creature immediately knows that it has been afflicted with some condition and must make 3 more successful saves at the end of each of its turns to stop it.

ALTERNATIVELY, you can choose for the contagion to lie dormant, target still has disadvantage on chosen ability BUT takes no damage and has to repeat the saving throws until it gets 3 successes or 3 failures. On 3 successes the target is back to their optimal condition. On 3 failures the target is afflicted for 7 days and has the poisoned condition. Choosing the alternative of this feature does not allow the target to detect anything is wrong with them. HOWEVER, the Detect Poison and Disease spell can find the affliction. If the Lesser Restoration spell, Greater Restoration spell, Heal spell or Lay on Hands is cast to remove the affliction, the infected must make the CON save again or keep their Poisoned condition until they otherwise make the save or last the 7 days.

Children of the Night

Once per long rest can cast Summon Swarm (Swarm of Rats, Swarm of Bats, Swarm of Ravens) without expending a spell slot. Additional swarms are unlocked at 8th, 10th, 12th and 14th levels.

Level 6: Viral Agent

The cancer mage can then attempt to infect a target with his viral agent, using his contagion ability. If successful, the viral agent can tell the cancer mage telepathically what its host experiences. In all other respects, the viral agent is a normal disease; if the victim fights it off or Lesser Restoration spell, Greater Restoration spell, Heal spell or Lay on Hands is used, the disease and the viral agent die. The cancer mage regains his Intelligence every Long Rest or when the viral agent dies. The cancer mage can create as many viral agents as he is willing to devote Intelligence points toward.

Pestilent Rebirth

The Virus you infect a creature with takes full control of their body upon death. You command your Viral Agent as you did before using all the stats or abilities of the dead person or monster it controls. Every hour this undead exists, the decay of death takes away some of its health. (1d8 Decay damage)

Whenever you create an undead using this class feature, it has additional benefits:

• The creature’s hit point maximum is increased by an amount equal to your wizard level.
• The creature regains 50% HP of this new total.
• The creature adds your proficiency bonus to its damage rolls.

Level 10: Insect Plague

Swarming locusts fill a 20-foot-radius Sphere centered on a point you choose within range. The Sphere remains for the duration, and its area is Lightly Obscured and Difficult Terrain.

When the swarm appears, each creature in it makes a constitution saving throw, taking 4d10 Piercing damage on a failed save or half as much damage on a successful one. A creature also makes this save when it enters the spell’s area for the first time on a turn or ends its turn there. A creature makes this save only once per turn. 

ALTERNATIVELY, if you choose yourself as the point of origin, you allow the swarm to surround your body dealing no damage to you and instead gain Insect Armor, giving you a +4 natural AC bonus. However, the cancer mage cannot wear armor heavier than light armor and keep this benefit.

Disease Form

The cancer mage may adopt his disease form as a standard action, attempting to infect a living creature within 100 feet. A potential victim must succeed at a CON save (DC 20) to avoid being infected with the cancer mage. Any creature that comes within 10 feet of a cancer mage in disease form is subject to the saving throw to resist infection as well. Once inside an infected host, the cancer mage deals 1d100 damage per day after a 24-hour incubation period. The victim has no way of knowing that the cancer mage is anything other than a normal illness.

The cancer mage travels with the victim, aware of whatever the victim is aware of. A number of times per day equal to the cancer mage's Charisma modifier, the cancer mage can attempt to force the victim to take one round's worth of actions of the cancer mage's choosing; a successful WIS save (DC 18 + cancer mage's WIS modifier) allows the victim to resist this. While controlling his victim, the cancer mage has access to all skills, feats, spells, and special abilities of the host, plus any of his own that he can use in his disembodied state.
The cancer mage can leave his host at any time, allowing the victim to recover normally. The victim can also attempt to force the cancer mage out by making CON saves as if the cancer mage were a normal disease. Heal checks help the victim as they would normally. The Lesser Restoration spell, Greater Restoration spell, Heal spell, or Lay on Hands drops the cancer mage to 0 Hit Points if he fails a CON save (DC 15 + CON modifier). Succeeding or failing the saving throw, forces the mage to reassume his material form adjacent to the victim. (The mage can make Death Saving throws to not outright die.)

A cancer mage can remain in disease form as long as he desires, or he can return to his natural form and be done with the disease form for that day. While in disease form, the cancer mage does not need to eat, sleep, or drink, and he does not age. A cancer mage could remain dormant in a room for a hundred years, then take his material form or infect a new victim.