In 5e, most damage heals naturally over a long rest. Some monsters however have abilities or attacks that cause permanent damage, for example permanent ability damage, hit point maximum reduction, petrification, removal of a vital organ or turning characters into undead that are not easily brought back to life.

By permanent I mean: any effect caused by a monster, that is not cured by a long rest or successful saving throws over several days, but that requires magical restoration.

This makes these monsters potentially much more dangerous than their challenge rating suggests, especially if the magical restoration that is needed is at a level that is much higher than what the player characters can be expected to have access to. Which monsters in the Monster Manual can cause such effects?

(You can also include monsters from other published sources if you like, I am not asking for it to keep the list bounded and complete; if you do, please indicate where they are from)

Qualifications

• Death is Normal. Almost any monster can cause death by dealing normal damage leading to failed death saves. This may lead to permanent death in the early levels, but this definition of permanent would mean we would include nearly every monster in the book. And even at earliest levels, healing spells and stabilization are available to the players to make a single player death less likely, as long as someone in the party still stands and can help. I'm not interested in that, normal damage dealing does not count here.

• Permanent is relative. Most permanently harmful effects can be undone by curative or life-restoring magic. If there is a way to undo the effect, at your option include examples of the lowest level abilities that could undo the effect, so the DM can see at which level the player characters could overcome it.

• Escalations count. Some monsters have effects that only trigger when nothing is done about earlier, less harmful effects. In this case include the answer to both stages