Create Plague

Make a melee spell attack against a creature within your reach. On a hit, the target is hit with a blast of poison. The target becomes poisoned. While poisoned, the target take’s 1d8 poison damage per turn. (You can then focus in or let loose.)

Focus: At the end of each of the targets turns, the target must make a CON saving throw, if the target succeeds on three of these saves then you and your target are inflicted with Spell-Link. 
Spell-link: you and the target are affected by the focus effect of this spell and trade the effect until failure. Upon failure the person who failed all three CON save throws is given Eternal Plague. 

Let Loose: you can inflict one of the diseases below (except eternal plague) on your target, but they lose the poison, without the cost of a spell slot. But you can only use this version of the spell 3 times before you have to long rest. Or you will be inflicted with Spell Fatigue.

Spell Fatigue: any and all spells you cast will deal damage based on what level it is. (1 damage per level of spell (1st level one damage, 9th level nine damage)) 

Blindness: The creature’s mind overheats and becomes blinded. It has disadvantage on wisdom checks, and wisdom saving throws.

Weakness: The creature’s body is severely weakened. It has disadvantage on strength checks, strength saving throws, and attack rolls that use strength.

Leprosy: The creature’s flesh starts to decay rapidly, they have disadvantage on charisma checks, and charisma saving throws. Also if it is a construct ,undead, spirit it is not affected. And if they are a normal creature, after 5 turns if the spell has not been dispelled by greater restoration, the creature dies. 

Migraines: The creature’s mind begins to overheats and hurts profusely. The creature has disadvantage on intelligence checks and intelligence saving throws, and the creature behaves as if it is under the confusion spell during combat.

Fever: The creature starts to have hot and cold flashes and starts to shake uncontrollably. The creature has disadvantage on Dexterity checks and Dexterity saving throws, and attacks rolls that use dexterity. 

Hemorrhaging: The creature begins to bleed uncontrollably. The creature has disadvantage on Constitution checks and Constitution saving throws. When using this disease you gain Spell Combo. In addition the creature has to make roll of more then ten or it falls to the ground and can’t get up.

Combo Spell: You can not gain this effect if you use Eternal plague. You can cast a second spell of 5 level or lower. 

Lupus: The creature’s nervous system starts to shut down. The creature takes 2d10 psychic damage on each of its turn. If the creature’s health drops to 0 the body becomes inhabitable. 

Eternal plague: The target that is effected by this plague is affected by all the effects of the diseases above. This disease can not be heal by greater restoration. You can only use this disease once a week (10 days). If the creature dies to the plague all creatures within 30 feet (creatures chosen to be affected are chosen by the caster) are inflicted with eternal plague. 

When cast at 9th level you can use it without having to go into the Spell-Link stage.