What is the default Conquerors: Inheritance game rule in CK3?

Primary Heir: The Conqueror personality and bonuses will only be inherited by the Primary Heir on Succession. (patch 1.20.1.2)

Does changing the Conquerors: Inheritance rule disable achievements?

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

Game rule · Difficulty · AI

Conquerors: Inheritance

All Heirs
Primary Heir · default
No Inheritance

Conquerors: Inheritance is a game rule in Crusader Kings III (Difficulty and AI) with 3 settings: All Heirs, Primary Heir (the default) and No Inheritance. 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 Conquerors: Inheritance game rule in CK3?: Primary Heir: The Conqueror personality and bonuses will only be inherited by the Primary Heir on Succession. (patch 1.20.1.2)
Does changing the Conquerors: Inheritance rule disable achievements?: No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

3 settings

All Heirs
The Conqueror personality and bonuses will be inherited by all Heirs that end up as Independent Rulers on Succession.
default_conquerors_inheritance
Primary Heir
Default
The Conqueror personality and bonuses will only be inherited by the Primary Heir on Succession.
The script checks it once, in 1 place
Story cycles 1
- Conqueror
Every has_game_rule = primary_conquerors_inheritance in game/common and game/events, by the top-level object it sits in.
primary_conquerors_inheritance
No Inheritance
The Conqueror personality will not be inherited.
The script checks it once, in 1 place
Story cycles 1
- Conqueror
Every has_game_rule = no_conquerors_inheritance in game/common and game/events, by the top-level object it sits in.
no_conquerors_inheritance

Source: game/common/game_rules/00_game_rules.txt line 204 (key conquerors_inheritance), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.