When Geralf's Messenger dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter and a vigilance counter on it.
When Geralf's Messenger enters the battlefield, it triggers, causing an opponent to lose 2 life.
Activate Tayam by paying ({3} magic symbol) , removing a -1/-1 counter from Devoted Druid, and both counters from Geralf's Messenger.
Holding priority, repeat steps 3-5 one additional time.
Activate Phyrexian Altar by sacrificing Geralf's Messenger, adding one mana of any color.
Resolve Tayam's ability from step 5, causing you to mill three cards and returning Geralf's Messenger from your graveyard to the battlefield.
Repeat.
Results
Infinite colored mana.
Infinite death triggers.
Infinite ETB.
Infinite lifeloss.
Infinite LTB.
Infinite sacrifice triggers.
Infinite self-mill.
Put all permanents with mana value three or less from your graveyard and library onto the battlefield.