Breach the Multiverse by paying its mana cost, targeting any applicable target.
Holding priority, cast
Dualcaster Mage by paying its mana cost.
When
Dualcaster Mage enters, it triggers, targeting
Breach the Multiverse.
Holding priority, activate
Phyrexian Altar by sacrificing
Dualcaster Mage.
Resolve the
Dualcaster Mage trigger, copying
Breach the Multiverse, with the copy targeting
Dualcaster Mage.
Resolve the
Breach the Multiverse copy, returning
Dualcaster Mage from your graveyard to the battlefield.
Repeat from step 3.
Notes
Breach the Multiverse can put all creature cards from all graveyards and libraries onto the battlefield under your control infinitely. It will also infinitely mill all players.
Results
Infinite colored mana.
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.
Infinite recursion of creature cards in your graveyard.
Put all creature cards from each opponent's library onto the battlefield under your control.