In Rain of Fear:
This is the main encounter in the Heart of Fear mini-raid event "Darkness" (see this quest entry for details).
In Kor-Sha Laboratory:
No placeholder. Respawns hourly with Screaming Tormentor, remains up for exactly 20 minutes then despawns if not engaged. Killing the regular amygdalan do not seem to impact the spawn timer, killing him or Screaming Tormentor will cause him to respawn one hour from the time of death.