Aghast

The Aghast are all that remains of once-mortal followers of the divinity of disease. Deemed as expendable, failures, traitors, or 'inadequate for their purposes,' they have been warped into hollow shells of their former selves, serving as near-immortal vectors. 

Appearance

Aghast maintain some traits of their living forms, appearing as reflections of their former selves, though warped and disfigured by their mutation and subsequent symbiosis with the Bitterblack fungus (more in Relationship with Bitterblack). As a result, they appear withered and gaunt, with elongated limbs and digits, their teeth sharpened with gums receded, and their greyish skin blistered and scattered with bruise-coloured boils. Their blood is as dark as ink and highly infectious. In older Aghast, small mushrooms of the Bitterblack fungus may sprout from their hide, producing a small cloud of spores that follows them. Veloxians appear to be immune to Ghastly Mutation, and therefore can only be transformed by the divine of disease themselves. 

Creation

An Aghast is thought to be created either by the divination themselves directly condemning a follower (or an insurgent), or by another Aghast via its Ghastly Mutation.

Ghastly Mutation

When a creature is affected by the Aghast's Ghastly Mutation, it slowly begins its transformation into becoming another Aghast. The process takes exactly six days, with new symptoms appearing each day. The Ghastly Mutation is regarded as a magical disease, but its severity increases depending on how far along it is - the more advanced the disease, the harder it is to cure.

1. The affected creature experiences a slight fever and a small, pale, spotted rash at the infection site that may be identified with a DC 18 Medicine check. However, the mutation also causes their senses to sharpen, giving them an additional +1 to Perception checks. It can easily be cured with Lay on Hands or a similar spell.
2. The creature experiences insomnia, sweating, light sensitivity, and general paranoia. The rash spreads, and may be noted with a DC 16 Medicine check. The mutation further improves their senses, granting them Darkvision up to 30ft if they didn't have it already, or a further 30ft if they did. It can be cured with Lesser Restoration, or a spell of greater power.
3. The creature experiences cravings for spoiled or rotten food. They feel uncomfortable in their own skin, feeling a horrible itch, and may fantasize about peeling it off. They have a general distrust of others, except for other infected. The mutation increases their aggressiveness, causing their Unarmed Strikes to deal an extra 3 points of damage. Their symptoms may be identified with a DC 14 Medicine check, and can still be cured with Lesser Restoration, or something of similar/greater power.
4. The creature can no longer sleep, or benefit from a Long Rest. Their distrust grows, and they choose to distance themselves from others, especially the healthy, and are more likely to refuse treatment. They experience severe mood swings between euphoria and despair, develop Dermatodaxia (biting or gnawing at one's own skin, especially around the fingers), and will avoid any and all hygiene practices. Their powers of perception increase, granting them Blindsight and Tremorsense to 30ft. Can be cured with Greater Restoration or something of similar/greater power.
5. The mutation is almost complete. The creature's tissues begin to decay, and they start losing memories and facets of their personality. They experience an undeterrable desire to find Bitterblack, believing it to be the only thing that can serve as a cure to their discomfort. If opposed, they may attack allies, no longer seeing friends in those that are free of disease. Dark, bruise-coloured boils appear on their skin, that are especially sensitive to sunlight. They may spread the mutation themselves through contact with their blood or other bodily fluids. If they are at any point reduced to 0 HP and killed, they will be revived as an Aghast the following dawn, unless their body is incinerated. An infected individual 5 days into the process can be cured by a Heal spell or something of similar/greater power.
6. The infected will go to any means necessary to find Bitterblack before their transformation is finished. Their pulse and breathing slow to a halt, and they no longer remember who they used to be. Their entire awareness consists of: finding and growing Bitterblack, spreading the mutation, and devoting themselves to the divine of disease. They will only hold back in the presence of those infected with the mutation or another pathogen. The only cure at this stage is nothing less than a Wish spell. 

Relationship with Bitterblack

Dwelling in their master's temples within the Underdark, they are instinctively drawn to cultivating foul fungi known as Bitterblack, which appear to have something of a symbiotic relationship with the Aghast.

The Bitterblack is a subspecies of Violet Fungus, yet its fruiting body produces plumes of black spores that greatly reduce visibility. The fungus body is highly toxic if eaten, as are the spores if inhaled, or come in contact with eye tissues, causing violent hallucinations, feelings of despair, and confusion. 

Scholars suspect that this symbiosis involves the Aghast cultivating the fungus - reducing its natural competition, helping overcome natural barriers, and providing protection, and in exchange, the Bitterblack's spores infuse with the blood and flesh of the Aghast. This not only makes the Aghast's flesh toxic, but actively protects it from further decomposition by other species, including other fungi, and sustains the diseases they carry, despite the host body being long dead. As a result, unlike other forms of undead, Aghast can exist for centuries, if not millennia, without their remaining tissues rotting away. Some plague specialists therefore actively study them, as they serve as hosts to ancient pathogens that could pose potentially catastrophic threat to life, should they ever make it to the surface.

Strengths and Weaknesses

Though their symbiosis with Bitterblack shields them from necrotic and poison damage, their dry tissues are susceptible to cold and fire damage, and they are sensitive to sunlight. Their skin, blood, and other bodily fluids are highly poisonous, and can trigger the Bitterblack's toxic effects if it manages to get under the skin. The mutation process strips them of their identity, humanity, and personality, their brain essentially reduced to a mush of rotting flesh and fungus, therefore they are immune to being Charmed or Frightened, and resist psychic damage. Though they can be made invisible, they have a distinctive odor, and their Terrifying Aura makes it possible for creatures to approximate their position if they are within 10ft.