The Fallen is an enemy that appears in the Northern Ruins in 600 AD and 1000 AD. This enemy, along with Flesh Reaper and Soul, can be encountered in the present if they are not killed in the past. You can fight them again in the past even after killing them in the present. But, they cannot be encounter vice versa.
Note: If a Soul is with a Fallen, the Soul will posses the Fallen and it would merge to become a Fallen + Soul.