Cavalier of Night and Altar of Dementia on the battlefield.
Steps
Activate Altar of Dementia by sacrificing Cavalier of Night.
When Cavalier of Night dies, it triggers, returning Activated Sleeper from your graveyard to the battlefield as a copy of Cavalier of Night.
When Activated Sleeper 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.
Activate Altar of Dementia by sacrificing Activated Sleeper.
When Activated Sleeper 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 Activated Sleeper enters the battlefield, it triggers; choose not to sacrifice a creature.
Resolve the Altar of Dementia ability from step 5.