set_house_head_effect

set_house_head_effect = {
	$NEW_HEAD$ = { save_scope_as = new_head }
	$OLD_HEAD$ = {
		save_scope_as = old_head
		if = {
			limit = {
				any_held_title = { is_noble_family_title = yes }
			}
			random_held_title = {
				limit = { is_noble_family_title = yes }
				save_scope_as = nf_title
			}
		}
	}
	create_title_and_vassal_change = {
		type = granted
		save_scope_as = change
		add_claim_on_loss = no
	}
	scope:nf_title = {
		change_title_holder_include_vassals = {
			holder = scope:new_head
			change = scope:change
		}
	}
	resolve_title_and_vassal_change = scope:change
	scope:new_head.house = { set_house_head = scope:new_head }
}

As written in game/common/scripted_effects/03_dlc_fp3_scripted_effects.txt.