Manuel II Palaiologos
Callistus II Xanthopoulos or Xanthopulus (; ? – after 1397) was a Byzantine Hesychast monk and spiritual writer who reigned as Ecumenical Patriarch of Constantinople in 1397. He was Patriarch through the reign of the Byzantine Emperor Manuel II Palaiologos, and through his short Patriarchal reign Constantinople was under siege by the Ottoman Sultan Bayezid I.