When Dualcaster Mage enters the battlefield, it triggers, copying Thunderous Debut.
Activate Altar of Dementia by sacrificing Dualcaster Mage.
When Dualcaster Mage dies, Mortuary triggers, putting Dualcaster Mage on top of your library from your graveyard.
Resolve the Altar of Dementia ability, causing target player to mill cards equal to Dualcaster Mage's power.
Resolve the copy of Thunderous Debut, causing you to look at the top twenty cards of your library and put Dualcaster Mage and up to one other creature card onto the battlefield.
Repeat from step 3.
Results
Infinite death triggers.
Infinite ETB.
Infinite LTB.
Infinite magecraft triggers.
Infinite mill.
Infinite sacrifice triggers.
Infinite self-mill.
Put all creature cards from your library onto the battlefield.