When the land enters the battlefield, it triggers.
Holding priority, activate
Altar of Dementia by sacrificing the land.
When the land dies,
Mortuary triggers, putting the land on top of your library from your graveyard.
Resolve the
Altar of Dementia ability, causing target opponent to mill cards equal to the land's power.
Resolve the land's trigger from step 3, causing you to put any other land onto the battlefield from your hand, draw a card, and then decline to put another land onto the battlefield.