When you take this Oath at level 1, you gain the following Channel Oath option:

As an Action, you cover your body in thick, fetid tumors for 3 turns, reducing incoming damage by an amount equal to your Charisma Modifier. A creature that attacks you with a melee attack causes a tumor to burst, inflicting 1d4 Necrotic damage on the attacker and possibly infecting it for 1 round, reducing their Constitution score by 1 and imposing Disadvantage on Constitution Saving Throws.

At 3rd level, you gain an additional Channel Oath ability:

 Your touch inflicts disease. Make a melee spell attack against a creature within your reach that isn’t a Construct or Undead. On a hit, the creature becomes Diseased and is Incapacitated for 1 minute. At the end of each of the diseased target's turns, the target can make a Constitution Saving Throw. On a success, the target is no longer Diseased, and the effect ends.

Since this ability induces a natural disease in its target, any effect that removes a disease or otherwise ameliorates a disease's effects apply to it.

You also gain the following Channel Oath option at 3rd level:

 You can use your Channel Oath to weaken a creature against entropic energies. As an Action, you can force a creature of your choice that you can see within 9m of you to make a Constitution Saving Throw. On a failed save, for the next minute, the creature takes an additional 2d6 damage the first time it takes Necrotic damage each turn. During this time, the creature loses Resistance to Necrotic damage if it has it.

Starting at 7th level, you emit an aura of contagion and virulence to a range of 3m. When a creature within your aura makes an attack roll or a Saving Throw, you can give that roll Disadvantage as a reaction. At 18th level, the range of this aura increases to 9m.

At 15th level, once per Long Rest when you take damage which would reduce you to 0 hit points, you drop to 1 hit point instead.

 In addition, if you are killed, your corpse violently explodes in a shower of pus and gore, dealing 8d6 Necrotic damage to creatures within 6m of you that fail a Constitution Saving Throw.

At 20th level, you become an avatar of plague, which gives you the following benefits:

• You have Resistance to Necrotic damage and are Immune to Poison damage and the Poisoned condition.

• Your hit point maximum can't be reduced.

In addition, enemies who start their turn within 1.5m of you, or who move within 1.5m of you on their turn, take Necrotic damage equal to your Charisma Modifier. Once a creature takes damage from this ability, it can't take damage from it again until the start of its next turn.

A living creature you choose within range breaks out into a cold sweat and must make a Constitution Saving Throw. On a failure, the cold sweat breaks into fever as the creature's skin becomes flushed and hot to the touch, and the creature takes 1d6 Fire damage at the start of its next turn for up to 1 minute or until your concentration breaks. On a success, the spell immediately ends.

While the spell persists, you can use a reaction at the start of an affected creature's turn to force the creature to take Fire damage again, increasing by 1d6 each time. The creature can use its action to attempt to cool itself, allowing it to attempt a Constitution Saving Throw again. On a success, the creature’s fever breaks and the spell ends.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the initial damage increases by 1d6 for each slot level above 3rd.