Activate Saffi Erikdotter by sacrificing itself, making it so the next time Gerrard dies this turn it will be returned to the battlefield.
Activate Altar of Dementia by sacrificing Gerrard.
Saffi Eriksdotter and Gerrard triggers.
Resolve the Saffi Eriksdotter trigger, returning Gerrard to the battlefield.
Resolve the Gerrard trigger, returning all artifact and creature cards that were put into your graveyard this turn from your graveyard onto the battlefield.
Resolve the Altar of Dementia's ability, causing target player to mill three cards.