During the period ending 31 December 2018, foreign citizens who attended 2018 FIFA World Cup™ matches may enter Russia visa-free, by presenting FAN ID (obtained either before or during FIFA) and valid passport. It is sufficient that the passport is valid throughout the journey. During the period you can enter Russia multiple times. Remember to exit country before the New Year.
For further information: https://www.fan-id.ru/index.html?locale=EN. Choose EN from the language option when you browse pages.