All permanents on the battlefield (Mirror Image as a copy of Cavalier of Night).
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Steps
Activate Altar of Dementia by sacrificing Mirror Image.
When Mirror Image dies, its last ability from Cavalier of Night triggers, returning itself from your graveyard to the battlefield as a copy of Cavalier of Night.
When Mirror Image enters the battlefield, its first triggered ability from Cavalier of Night triggers; choose not to sacrifice a creature.
Resolve the Altar of Dementia ability from step 1.