reclaim_britannia_decision_effect

reclaim_britannia_decision_effect = {
	every_sub_realm_county = {
		limit = {
			NOR = {
				culture = { has_cultural_pillar = heritage_goidelic }
				culture = { has_cultural_pillar = heritage_brythonic }
			}
			title_province = { geographical_region = world_europe_west_britannia }
		}
		custom = british_isles.0001.custom
		add_county_modifier = {
			modifier = reclaim_britannia_culture_modifier
			days = 3650
		}
	}
	capital_county = {
		set_county_culture = root.culture
	}
	if = {
		limit = {
			culture = { has_cultural_pillar = heritage_goidelic }
		}
		give_nickname = nick_the_tuatha_de_danann
	}
	else = {
		give_nickname = nick_pendragon
	}
}

As written in game/common/scripted_effects/00_decisions_effects.txt.