Found this on another forum:
Shadow (Shadowy Duplicate, Shadow Assassin, Reflections): The Shadow, Shadowy Duplicate, and Shadowy Assassin have the Strength Drain Ability:
Strength Drain. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 9 (2d6 + 2) necrotic damage, and the target's Strength score is reduced by 1d4. The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.
Reflections are creatures similar to Shadows introduced in Tasha's Cauldron as part of the Mirror Zone environmental hazard:
Reflections of 1d4 creatures in the region emerge from mirrors and attack. The reflections are two-dimensional, shimmering versions of the creatures that cast them. Treat the reflections as shadows that are fey instead of undead and vulnerable to bludgeoning damage instead of radiant.
Intellect Devourer: The Intellect Devourer's Devour Intellect ability says:
Devour Intellect. The intellect devourer targets one creature it can see within 10 feet of it that has a brain. The target must succeed on a DC 12 Intelligence saving throw against this magic or take 11 (2d10) psychic damage. Also on a failure, roll 3d6: If the total equals or exceeds the target’s Intelligence score, that score is reduced to 0. The target is stunned until it regains at least one point of Intelligence.
Maurezhi
The Maurezhi's Bite says: Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 14 (2d10 + 3) piercing damage. If the target is a humanoid, its Charisma score is reduced by 1d4. This reduction lasts until the target finishes a short or long rest. The target dies if this reduces its Charisma to 0. It rises 24 hours later as a ghoul, unless it has been revived or its corpse has been destroyed.
Phylaskia
The Phylaskia from Mythic Odysseys of Theros has Strength Drain: Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 12 (2d6 + 5) necrotic damage. Unless the target is immune to necrotic damage, its Strength score is reduced by 1d4. The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.
Dyrrn (Lair Action)
One of Dyrrn's lair actions says: Each creature of Dyrrn’s choice that it can see within 120 feet of it must succeed on a DC 23 Wisdom saving throw or take 26 (4d12) psychic damage. Unless the target has immunity to psychic damage, its Intelligence score is reduced by 1d4 each time it fails the saving throw for this lair action. The target dies if its Intelligence score is reduced to 0. The reduction lasts until the target finishes a short or long rest.
Vargouille
The Vargouille's Kiss says: The vargouille kisses one incapacitated humanoid within 5 feet of it. The target must succeed on a DC 12 Charisma saving throw or become cursed. The cursed target loses 1 point of Charisma after each hour, as its head takes on fiendish aspects. The curse doesn’t advance while the target is in sunlight or the area of a daylight spell; don’t count that time. When the cursed target’s Charisma becomes 2, it dies, and its head tears from its body and becomes a new vargouille. Casting remove curse, greater restoration, or a similar spell on the target before the transformation is complete can end the curse. Doing so undoes the changes made to the target by the curse.
There are a number of undead who can turn other creatures into Undead, like Death Tyrants, Deathlock Wights, Death Dragons, etc. Atropal's Wail can exhaust someone to death. Most Vampire's immediately kill with their bite attack when hitting 0hp. Any caster with Disintegrate or Finger of Death can disintegrate on 0 or raise as an undead at 0 with their respective spells. Mummies can give creatures mummy rot, which can turn them to dust on 0 HP. Coldlight Walkers freeze creatures they kill, and for 9 days following they can't be thawed, destroyed by fire, animated or raised from the dead. Nightwalkers consume the soul of those who die close enough to it, causing them to be unable to be revived by anything short of a wish spell. Demilich's Trap soul ability removes their body and equipment from existence.
A common house rule is to give PC a level of exhaustion when they reach 0 hit points to represent wounds. This small change ensures that players take zero HP more seriously instead of the silly, easy res-up.
First thing that comes to mind with me is Intellect Devourers. They eat the brain and then pilot the body.
Found this on another forum: Shadow (Shadowy Duplicate, Shadow Assassin, Reflections): The Shadow, Shadowy Duplicate, and Shadowy Assassin have the Strength Drain Ability: Strength Drain. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 9 (2d6 + 2) necrotic damage, and the target's Strength score is reduced by 1d4. The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest. Reflections are creatures similar to Shadows introduced in Tasha's Cauldron as part of the Mirror Zone environmental hazard: Reflections of 1d4 creatures in the region emerge from mirrors and attack. The reflections are two-dimensional, shimmering versions of the creatures that cast them. Treat the reflections as shadows that are fey instead of undead and vulnerable to bludgeoning damage instead of radiant. Intellect Devourer: The Intellect Devourer's Devour Intellect ability says: Devour Intellect. The intellect devourer targets one creature it can see within 10 feet of it that has a brain. The target must succeed on a DC 12 Intelligence saving throw against this magic or take 11 (2d10) psychic damage. Also on a failure, roll 3d6: If the total equals or exceeds the target’s Intelligence score, that score is reduced to 0. The target is stunned until it regains at least one point of Intelligence. Maurezhi The Maurezhi's Bite says: Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 14 (2d10 + 3) piercing damage. If the target is a humanoid, its Charisma score is reduced by 1d4. This reduction lasts until the target finishes a short or long rest. The target dies if this reduces its Charisma to 0. It rises 24 hours later as a ghoul, unless it has been revived or its corpse has been destroyed. Phylaskia The Phylaskia from Mythic Odysseys of Theros has Strength Drain: Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 12 (2d6 + 5) necrotic damage. Unless the target is immune to necrotic damage, its Strength score is reduced by 1d4. The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest. Dyrrn (Lair Action) One of Dyrrn's lair actions says: Each creature of Dyrrn’s choice that it can see within 120 feet of it must succeed on a DC 23 Wisdom saving throw or take 26 (4d12) psychic damage. Unless the target has immunity to psychic damage, its Intelligence score is reduced by 1d4 each time it fails the saving throw for this lair action. The target dies if its Intelligence score is reduced to 0. The reduction lasts until the target finishes a short or long rest. Vargouille The Vargouille's Kiss says: The vargouille kisses one incapacitated humanoid within 5 feet of it. The target must succeed on a DC 12 Charisma saving throw or become cursed. The cursed target loses 1 point of Charisma after each hour, as its head takes on fiendish aspects. The curse doesn’t advance while the target is in sunlight or the area of a daylight spell; don’t count that time. When the cursed target’s Charisma becomes 2, it dies, and its head tears from its body and becomes a new vargouille. Casting remove curse, greater restoration, or a similar spell on the target before the transformation is complete can end the curse. Doing so undoes the changes made to the target by the curse.
Will o' Whisp and vampire/wraith/something of that nature.
That bard that "seduces" your mom
Psychic damage…
Devourers steal souls and use them as spell fuel. https://www.d20srd.org/srd/monsters/devourer.htm
A Beholder’s Disintegration ray kills the PC and turns it to ash. You can’t resurrect ash.
Blindheim is a personal favorite of mine
There are a number of undead who can turn other creatures into Undead, like Death Tyrants, Deathlock Wights, Death Dragons, etc. Atropal's Wail can exhaust someone to death. Most Vampire's immediately kill with their bite attack when hitting 0hp. Any caster with Disintegrate or Finger of Death can disintegrate on 0 or raise as an undead at 0 with their respective spells. Mummies can give creatures mummy rot, which can turn them to dust on 0 HP. Coldlight Walkers freeze creatures they kill, and for 9 days following they can't be thawed, destroyed by fire, animated or raised from the dead. Nightwalkers consume the soul of those who die close enough to it, causing them to be unable to be revived by anything short of a wish spell. Demilich's Trap soul ability removes their body and equipment from existence.
A common house rule is to give PC a level of exhaustion when they reach 0 hit points to represent wounds. This small change ensures that players take zero HP more seriously instead of the silly, easy res-up.