Aven Eternal by paying its mana cost; it enters with a vigilance counter due to
Tayam.
When
Aven Eternal enters, it triggers, creating a 0/0 creature token with a +1/+1 counter on it as well as a vigilance counter due to
Tayam.
Declare that you are activating
Tayam's ability, so state-based actions will not be checked until the ability is activated.
While paying that ability's cost, activate the creature token using the
Wall of Roots mana ability from
Agatha's Soul Cauldron by putting a -0/-1 counter on it, adding ({G} magic symbol) .
Complete the activation by paying ({3} magic symbol) and removing a vigilance counter from each of the two creatures and a -0/-1 counter from the Army token.
Holding priority, activate
Phyrexian Altar twice by sacrificing
Aven Eternal and the creature token, adding two mana.
Resolve
Tayam's ability, milling three and returning
Aven Eternal from your graveyard to the battlefield with a vigilance counter on it.
Repeat from step 2.
Notes
This order of operations (and the additional ({2} magic symbol) starting mana) are necessary because you must activate mana abilities before paying costs, and so you cannot use Ashnod's Altar to get mana from the creature you need to remove counters from.
Results
Infinite creature ETB.
Infinite creature LTB.
Infinite creature sacrifice triggers.
Infinite death triggers.
Infinite self-mill.
Bracket Tag
This combo has been estimated to be Exhibition (Bracket 1+).