What is the default Random Harm Target game rule in CK3?

AI Only: Events that kill, incapacitate, or hurt the Health of Characters at random can happen to the AI only. (patch 1.20.1.2)

Does changing the Random Harm Target rule disable achievements?

No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Difficulty · Health

Random Harm Target

Anyone
Player Only
AI Only · default

Random Harm Target is a game rule in Crusader Kings III (Difficulty and Health) with 3 settings: Anyone, Player Only and AI Only (the default). 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 Target game rule in CK3?: AI Only: Events that kill, incapacitate, or hurt the Health of Characters at random can happen to the AI only. (patch 1.20.1.2)
Does changing the Random Harm Target rule disable achievements?: No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

3 settings

Anyone
Events that kill, incapacitate, or hurt the Health of Characters at random can happen to any Character.
harm_target_all
Player Only
Events that kill, incapacitate, or hurt the Health of Characters at random can happen to players only.
harm_target_player_only
AI Only
Default
Events that kill, incapacitate, or hurt the Health of Characters at random can happen to the AI only.
harm_target_ai_only

Source: game/common/game_rules/00_game_rules.txt line 116 (key random_harm_target), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.