The Plague

 The Plague is a dark god, and has pacts with many mortals to spread disease and therefore his domain. You are one of these lucky (or unlucky) mortals. You serve this Hand of Death with your life.

Infected Weapon

Your pact has gained power, and you can now make any weapon you use to attack do an extra 1d8 poison damage.

Plaguebound Soul

You are able to summon a plague spirit that has 10 health, immunity to poison damage, and does 1d6 poison damage.

Venomous Retaliation

When you are attacked, you can use a reaction to deal back half of that damage as poison damage to your attacker. This works if the attack kills you, but not if you were attacked from a distance or the attacker is immune to poison.

The Avatar of Disease

You can summon a Huge avatar that is made of first-grade poison and Plague Essence. When you summon it, your body and everything on our person disappears, and your consciousness and mind is transferred to the avatar. You can move 30 ft, have 80 health, and have a Large blade made of poison. You can attack using the blade and roll a d20 to hit. If you hit, you do 3d10 damage. All damage you do is Poison damage. You can do this once per Long Rest. If the avatar is killed, your body and everything on your person reappears where it was before and your consciousness and mind are transferred back to it. You additionally lose all status conditions that you had before.