create_custom_duchy_effect

create_custom_duchy_effect = {
	save_scope_as = root_scope
	custom_tooltip = create_new_duchy
	show_as_tooltip = {
		primary_title = {
			custom_tooltip = create_custom_duchy_de_jure_changes
		}
	}

	hidden_effect = {
		save_scope_as = founder
		primary_title = {
			save_scope_as = old_title
		}

		create_dynamic_title = {
			tier = duchy
			name = NEW_CREATED_TITLE_NAME
		}
		create_title_and_vassal_change = {
			type = created
			save_scope_as = change
			add_claim_on_loss = no
		}

		scope:new_title = {
			change_title_holder = {
				holder = root
				change = scope:change
			}
		}

		resolve_title_and_vassal_change = scope:change

		scope:new_title = {
			set_de_jure_liege_title = scope:old_title.kingdom
		}
		scope:old_title = {
			set_de_jure_liege_title = scope:new_title
		}

		scope:new_title = {
			set_coa = scope:old_title
			set_color_from_title = scope:old_title
			set_capital_county = scope:old_title.title_capital_county
		}
		set_primary_title_to = scope:new_title

		trigger_event = major_decisions.1101
	}
}

As written in game/common/scripted_effects/00_major_decisions_scripted_effects_3.txt.