Cast Dance of Many by paying ({U} magic symbol) ({U} magic symbol) .
Dance of Many enters the battlefield, creating a token copy of Magnanimous Magistrate.
Activate Altar of Dementia by sacrificing Dance of Many.
Dance of Many, Magnanimous Magistrate, and the token Magnanimous Magistrate trigger.
Resolve the nontoken Magnanimous Magistrate trigger, declining to act.
Resolve the token Magnanimous Magistrate trigger, removing two reprieve counters from it and returning Dance of Many from your graveyard to the battlefield.
Dance of Many enters the battlefield, creating a token copy of Magnanimous Magistrate.
Resolve the Dance of Many trigger, exiling the first Magnanimous Magistrate token.
Repeat from step 3 any number of times.
Resolve all Altar of Dementia abilities, milling any number of players any amount of cards.
You may sacrifice and return any number of creatures with mana value 3 or less in your graveyard any number of times.
Results
Infinite blinking of creatures with mana value 3 or less on the battlefield, in graveyard, and in library.