What is the default Easter Egg Characters game rule in CK3?

Default: Easter Egg characters will be present at the start of the game. (patch 1.20.1.2)

Does changing the Easter Egg Characters rule disable achievements?

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

Game rule · Historicity

Easter Egg Characters

Default
None

Easter Egg Characters is a game rule in Crusader Kings III (Historicity) with 2 settings: Default (the default) and None. 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 Easter Egg Characters game rule in CK3?: Default: Easter Egg characters will be present at the start of the game. (patch 1.20.1.2)
Does changing the Easter Egg Characters rule disable achievements?: No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

2 settings

Default
Default
Easter Egg characters will be present at the start of the game.
default_easteregg_characters
None
No Easter Egg characters will be present at the start of the game.
The script checks it once, in 1 place
On actions (the game's own hooks) 1
- on_game_start_after_lobby
Every has_game_rule = no_easteregg_characters in game/common and game/events, by the top-level object it sits in.
no_easteregg_characters

Source: game/common/game_rules/00_game_rules.txt line 1582 (key easteregg_characters), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.