Which monsters can scare players by harming their PCs long-term? [closed]

Question

In 5e, most damage heals naturally over a long rest. Some monsters however have abilities or attacks that cause long-term harm, harm that is not cured by a long rest or by saving throws over time. Harm that requires magical restoration.

This makes these monsters potentially much more dangerous than their challenge rating suggests, if the magical restoration that is needed is unavailable to the player characters. It could leave the characters crippled for a long time. If the players know this, this can makes encounters with such monsters a stressful, high tension experience.

Which monsters in the Monster Manual cause such effects?

(The question is about the Monster Manual to keep the list bounded and complete; if you do include monsters from other officially published sources, please indicate where they are from)

Qualifications

Damage is Normal. Almost any monster can cause death by dealing damage leading to failed death saves. If your character is level one, every monster is scary. But even at earliest levels, healing spells and stabilization are available to the players to prevent this, as long as someone in the party still stands and can help. So normal damage dealing is not sufficient to be included.

Long-term. Nearly all harmful effects can be undone by powerful-enough magic. The question is about effects that do not go away by themselves. (Optionally, include examples of the lowest level abilities that could undo the effect. Some monsters have secondary effects that only trigger when nothing is done about the first stage in time. If you include guidance on how to end the effect, best include the answer to both stages.)

I made the answer a Wiki, so you should be able to add to it and expand it.

Answer 1

Scare, do not frustrate

There is a reason all these scary aspects in 5e were toned down compared to the early editions of D&D: while the fear of it may create exitement and intense situations, having your PC mutlilated, level-drained, or their beloved magic item destroyed is not fun. So, if you use these monsters, especially the ones to which the PCs have no answers, it is a good idea to include solutions to undo the their effects in your adventures. If you are confronting a level 2 group with a Basilisk, hide a scroll of Greater Restoration somewhere, or have a friendly cleric live in their hometown who is able to help. This can well be the source for new adventures they undertake to pay off their debt.

Problem Monsters

Especially problematic monsters cause permanent effects long before the player characters can revert them, or that cannot be undone at all. Some of the main culprits:

The Intellect Devourer appears level appropriate for a level 2 party, but can turn a character into a mental vegetable, which needs spells not available before level 9 to fix. If it gets your brain, you need Resurrection, a level 13 spell. And most characters are likely to fail saves against its attack. This is maybe the most scary monster in the entire book. (It is especially deadly to confront a party with a pack of these. The monsters can sense the PCs within 300 feet and have high movement so they can likely ambush them. It sounds as if a level four party should be able to handle it, but there is a high chance to kill the party that way).

The Gibbering Mouther, also for a level 2 party, might resorb a fallen character and require a Wish to undo it. Thankfully, it is a bit unlikely that he gets to that point.

The Basilisk appears as an appropriate opponent for level 3 groups, but can petrify, which again needs a level 9 caster to undo with Greater Restoration.

The Wraith, a mid-level opponent, can potentially turn a character into a Specter. Unless the group immediately Revivify the victim, they will need access to a True Resurrection or Wish. The common tactic of killing the undead form of their friend, then reviving him will not work, as killing a Specter destroys the soul forever.

The Githzerai Zert, also mid-level foe at CR 6, can Plane Shift a character. Until the group gets access to planar travel themselves on level 13, this essentially removes the victim from play for good.

The Mind Flayer is living up to its scary image. It can extract your brain when you fight them around level seven, and short of a Resurrection, a level 13 spell, you are out of luck.

At the top end, a Demilich can trap your soul, and if you do not do anything about it for a day, destroy it forever, so not even a Wish can get it back.

Solutions by character level

Here is a brief overview with what characters can deal at a given level:

On level one, death is permanent. You do not have the funds or abilities to revive a dead comerade. Other than wounds, you can undo nothing. So everything is scary. Level one is closest to real live. (Only Paladins already can heal disease or neutralize poison here).

On level three, characters gain access to Lesser Restoration. Now they can heal diseases and neutralize poisons (and remove permanent blindness and deafness, although that rarely happens).

On level five, characters gain access to Revivify and Remove Curse to undo recent death and cure lycantrophy infections and mummy rot. They have more money to buy magical and divine aid. They still need to get help from powerful NPCs to fix petrification or permanent ability damage or to raise the dead.

On level nine comes a big step up with access to Greater Restoration and Raise Dead. Petrification, permanent ability score damage and permanent hit point maximum reduction can be undone. This is after a long period from level five, and around this level many campaigns start to wind down.

On level thirteen they get access to Plane Shift, Astral Projection, Resurrection and Regenerate so if a monster sends them to hell on a failed save, they are not gone forever from the campaign. And if they lose their limbs, brains or internal organs, they can get them back and live.

On level seventeen, players finally unlock the top end, with True Ressurection and Wish. Nearly nothing is permanent now, you can undo the worst character-mangling mishaps (and that is also exclusively how wish was being used by Gygax and Co. in 1e). Scary here are only the things that even a wish cannot fix, like your soul being destroyed.

Monsters Ordered by Challenge Rating

In cases where I am actively not sure if the countermeasure would work, I marked it with a question mark. Each of those would warrant a separate question. Restorative magic or solutions are given (in parentheses).

Challenge Rating 1/2

• Gas Spore - Spores (Lesser Restoration)
• Shadow - Strength reduction to 0 spawns shadow after 1d4h (Revivify corpse)
• Gray Ooze - permanently corrodes nonmagical weapons and armor
• Rust Monster - permanently corrodes non-magical metal weapons and armor

Challenge Rating 2

• Intellect Devourer - Int damage (Greater Restoration), remove brain (Resurrection)
• Gibbering Mouther - absorbed into mouther when killed by it (Wish?)
• Wererat - Lycanthropy (Remove Curse)
• Myconoid Sovereign - Animating Spores (kill and Revivify?)

Challenge Rating 3

• Werewolf - Lycanthropy (Remove Curse)
• Basilik - Petrification (Greater Restoration)
• Mummy - Mummy Rot (Remove Curse)
• Wight - hp max reduction, (Revivify), raise as zombie after 24 h (kill & Resurrection)

Challenge Rating 4

• Ghost - Possession, (Dispel Evil) or hitting down to 0
• Wereboar - Lycanthropy (Remove Curse)
• Weretiger - Lycanthropy (Remove Curse)
• Black Pudding - permanently corrodes nonmagical weapons, armor

Challenge Rating 5

• Gorgon - Petrification (Greater Restoration)
• Werebear - Lycanthropy (Remove Curse)
• Red Slaad - chaos phage (Lesser Restoration, after gestation Revivify)
• Wraith - Create Specter from freshly fallen foe (Revivify) [Note: if Specter dies, soul is destroyed forever]
• Otyugh - disesase (save every 24 h cures or Lesser Restoration)
• Night Hag - reduce hp max (Greater Restoration), on death soul is trapped

Challenge Rating 6

• Medusa - Petrification (Greater Restoration)
• Githzerai Zerth - Plane Shift (Plane Shift)

Challenge Rating 7

• Mind Flayer - Extract Brain (Resurrection)
• Blue Slaad - chaos phage (Lesser Restoration, or after transformation Wish)

Challenge Rating 8

• Githyanki Knight - permanently removes with Plane Shift (Plane Shift)

Challenge Rating 9

• Clay Golem - hp maximum Reduction (Greater Restoration)

Challenge Rating 10

• Aboleth - permanently dominates (taking damage gives resaves)
• Death Slaad - Plane Shift (Plane Shift)

Challenge Rating 11

• Djinni/Dao/Marid/Efreeti - permanently removes with Plane Shift (Plane Shift)

Challenge Rating 12

• Arcanaloth - Finger of Death zombifies (kill & Resurrection)

Challenge Rating 13

• Beholder - Petrification (Greater Restoration) and Disintegrate (True Ressurection or Wish)
• Rakshasa - Dream Curse (Remove Curse), Plane Shift (Plane Shift)
• Vampire - Bite & bury to raise as Vampire Spawn (kill and Raise Dead? or Wish)
• Shadow Dragon - Shadow Breath creates shadow on kill (Revivify on corpse)

Challenge Rating 15

• Mummy Lord - Mummy Rot (Remove Curse)

Challenge Rating 21

• Demilich - Energy Drain permanent Ability Damage, Trap Soul 24 h (no solution)
• Lich - Finger of Death zombifies (kill & Resurrection), Plane Shift (Plane Shift), Disintegrate (True Ressurection or Wish)

This is a Wiki. If I missed something or made a mistake somewhere, you can change it.