Soulshrieker

Undead are popular monsters for a reason: they're scary and there is no reason to feel bad for putting them down, since they are animated corpses.  Gamemasters don't even need to create some bizarre biome or food web for them in an attempt at Gygaxian Naturalism.

Just place them in a tomb, catacomb, or other abandoned place, and have at it.

Soulshriek by John Bolton

The Soulshrieker is a rare form of undead that is primarily the result of mortals that died in mourning and abject sorrow.  That tangible sorrow wells up and is projected from the Soulshrieker's oversized mouth, as a wail of piteous defeat that leaves targets stunned with their own sorrow and depression.  

Mechanically, the soulshriek is a beam-shaped breath weapon, appearing as coruscating purple and green light (how I picture a particle beam to look, which likely doesn't help).  It inflicts sonic, necrotic, and cold damage while also having a chance to stun its target.  Once stunned, the targets are then ripped apart with the Soulshrieker's claws.  

Mostly encountered alone, a group Soulshriekers (probably a family before death and undeath took them) is best fled from.  That said, jamming wax in your ears provides a substantial bonus to the save vs stun.  

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Enterprising necromancers have not only discovered how to create Soulshriekers, but have also found a way to mimic the Soulshrieker's ability, through a spell fittingly called Soulshriek.

This Necromantic spell lets the caster unhinge their jaw and turn their very soul into a weapon that deals sonic, necrotic AND cold damage to the unfortunates caught up in its cone; the spell emanates as a cone from the unhinged mouth, the interior of the mouth glowing white hot as the necromancer's very essence powers it.  This shriek is devastating to opponents within earshot, leaving them damaged, stunned, and if too close to the point of origin, deafened.

While one casting of Spellshriek can be devastating to its targets, it proves problematic for the caster, as part of their soul is now forever gone.  

Mechanically, this means that math is required.  Because base ten is simple, the caster gets ten castings of Soulshriek before they are forever gone. As the soul dwindles with repeated castings, the caster's body begins showing signs of spiritual decay.  

Ten Visual Signs of Spiritual Decay (effects are cumulative)

1. Skin pales, hair turns grey, loses interest in all sexual activity.
2. -1 strength.  
3. -1 constitution
4. -1 strength. Hair begins falling out.  
5. -1 constitution. Physically appears the same, but long rests are interrupted with nightmares. Wake in a cold sweat geased to scribe Soulshriek onto a half-dozen scrolls and leave them where they're likely to be found and copied.  Once the scrolls are written, the nightmares end (or the caster is used to them - either way, long rests are no longer affected by them).  Note that the scrolls never reveal the downsides of the spell.
6. -1 strength
7. -1 constitution. Lose interest in cleanliness.
8. -1 strength. Weight loss continues.
9. -1 constitution. A walking husk with eyes that glow with an inner, disturbing light. Easily mistaken for undead.
10. The lights in the eyes go out as the body collapses in a heap, open for possession or to become a true undead.  Cannot be raised or resurrected or even the target of speak with dead.

With the tenth casting, the soul is forever gone.  Without a soul, the caster's body withers entirely, and if not possessed by some random fiend or spirit, they rise up as a Soulshrieker in 24 hours.  

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While the more competent necromancers can avoid the worst effects of Soulshriek, lesser casters learn to their chagrin that Soulshriek is a living spell, a spell that demands to be cast, and will be cast.  

Once Soulshriek has been first cast, there is forever an emptiness in the soul, a cancerous emptiness that desires to grow.  As such, whenever the necromancer casts a spell, there is a chance that Soulshriek replaces the desired spell, even if Soulshriek isn't memorized - because it now resides within the caster.  

First, the spell cast has to be level 3 or higher.  Second, a save is required - just a flat DC 12 save.  With time, as Spellshriek is cast again and again, that DC increases by 1 with each casting.  So that after nine castings, it is a DC 20 save to not Soulshriek one final, glorious time.

It is whispered that Grandfather Favarro has discovered a means to purge Soulshriek from a caster, but given that its Grandfather Favarro, the cure is likely to be as bad as the disease.  It assuredly involves more than just high level remove curse, dispel magic, and greater restoration spells.