What is the default Empire Obscurity game rule in CK3?

On: De Jure Empires will be destroyed unless the holder controls at least 20% of its De Jure Counties. (patch 1.20.1.2)

Does changing the Empire Obscurity rule disable achievements?

No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Titles & Realm · Tweaks

Empire Obscurity

On · default
Off

Empire Obscurity is a game rule in Crusader Kings III (Titles & Realm and Tweaks) with 2 settings: On (the default) and Off. 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 Empire Obscurity game rule in CK3?: On: De Jure Empires will be destroyed unless the holder controls at least 20% of its De Jure Counties. (patch 1.20.1.2)
Does changing the Empire Obscurity rule disable achievements?: No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

2 settings

On
Default
De Jure Empires will be destroyed unless the holder controls at least 20% of its De Jure Counties.
The script checks it once, in 1 place
Events 1
- Obscurity of the …
Every has_game_rule = on_empire_obscurity_requirement in game/common and game/events, by the top-level object it sits in.
on_empire_obscurity_requirement
Off
Empires will persist regardless of how much De Jure land the holder controls.
off_empire_obscurity_requirement

Source: game/common/game_rules/00_game_rules.txt line 964 (key empire_obscurity_requirement), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.