In the Plane of Storms: Spawns during the encounter with Jeplak, Lord of Srerendi when someone in the raid group dies.
In the Ruins of Illsalin: (no data)
In Vergalid Mines: Vergalid's End: Part of the encounter with Goru Uldrock, the Reincarnate.
In The Well: No Freedom in Death: Spawns as part of the task "No Freedom in Death".
In the Convorteum: Part of "The Stone Warden" event (see this quest entry for event information and loot).
In Sepulcher West: Part of the raid event "Two Faces, One Vision" (see this quest entry for event information).
In Chapterhouse of the Fallen: Upper case "A Lost Soul" is a named known to spawn in the basement at location 450, 245, -222 (PH "an enraged willow wisp"). "Flame Spiral" is cast every 20 seconds on random target (but not most hated). "Aegis of the Fallen" (spell rune) is cast every 40 seconds. The lower case "a lost soul" is a common trash mob in the prison area.