What is the default Random Harm game rule in CK3?

Dangerous: Accidents, bad luck, and the mercurial whims of Fate may inflict Death, the Incapable Trait, and loss of Health at random. (patch 1.20.1.2)

Does changing the Random Harm rule disable achievements?

No: none of its 6 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Difficulty · Health

Random Harm

Dangerous · default
Illusion of Safety
Safe
Tragic
Spiteful
Tragically Spiteful

Random Harm is a game rule in Crusader Kings III (Difficulty and Health) with 6 settings: Dangerous (the default), Illusion of Safety, Safe, Tragic, Spiteful and Tragically Spiteful. None of its settings disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Random Harm game rule in CK3?: Dangerous: Accidents, bad luck, and the mercurial whims of Fate may inflict Death, the Incapable Trait, and loss of Health at random. (patch 1.20.1.2)
Does changing the Random Harm rule disable achievements?: No: none of its 6 settings carries the flag blocks_achievements in patch 1.20.1.2.

6 settings

Dangerous
Default
Accidents, bad luck, and the mercurial whims of Fate may inflict Death, the Incapable Trait, and loss of Health at random.
The script checks it once, in 1 place
Script values 1
- harm_game_rule_likelihood_value
Every has_game_rule = harm_dangerous in game/common and game/events, by the top-level object it sits in.
harm_dangerous
Illusion of Safety
Random Death or disabling events are much rarer. Till they aren't.
The script checks it once, in 1 place
Script values 1
- harm_game_rule_likelihood_value
Every has_game_rule = harm_illusion_of_safety in game/common and game/events, by the top-level object it sits in.
harm_illusion_of_safety
Safe
Random harmful events will not kill or cripple Characters. They may still die prematurely in other events, especially if in dangerous situations.
harm_safe
Tragic
Not only the Good die young — now you can too, with boosted chances of random harm events for everyone!
The script checks it once, in 1 place
Script values 1
- harm_game_rule_likelihood_value
Every has_game_rule = harm_tragic in game/common and game/events, by the top-level object it sits in.
harm_tragic
Spiteful
Fate focuses its cruel wrath on Characters of note: those with high Skills, inheritable Traits, notable achievements, and so on.
The script checks it 2 times, in 1 place
Script values 1
- harm_game_rule_likelihood_value
Every has_game_rule = harm_spiteful in game/common and game/events, by the top-level object it sits in.
harm_spiteful
Tragically Spiteful
Everyone's chance of random harm events is markedly increased, but most especially anyone of note.
The script checks it 2 times, in 1 place
Script values 1
- harm_game_rule_likelihood_value
Every has_game_rule = harm_tragically_spiteful in game/common and game/events, by the top-level object it sits in.
harm_tragically_spiteful

Source: game/common/game_rules/00_game_rules.txt line 95 (key random_harm), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.