Here is the full list of all 16 OverNight achievements.
Identify the Spirit in a session (any difficulty)
Identify the Valac in a session (any difficulty)
Identify the Haberdasher in a session (any difficulty)
Identify the Lucifer in a session (any difficulty)
Identify the Voodoo in a session (any difficulty)
Identify the Aim in a session (any difficulty)
Identify the Lilith in a session (any difficulty)
Identify the Phantom in a session (any difficulty)
Identify the Poltergeist in a session (any difficulty)
Identify the Wraith in a session (any difficulty)
Identify the Phase in a session (any difficulty)
Identify the Wisp in a session (any difficulty)
Identify the Candor in a session (any difficulty)
Identify the Mirex in a session (any difficulty)
Identify the Oblivion in a session (any difficulty)
Identify the Shade in a session (any difficulty)