Cast Grim Initiate by paying {R}.
Grim Initiate enters the battlefield, triggering Obsidian Battle-Axe, attaching it to Grim Initiate.
Activate Phyrexian Altar by sacrificing Grim Initiate, adding {R}.
Grim Initiate dies, triggering itself and Koll.
Resolve Grim Initiate's trigger, amassing 1.
Resolve Koll's trigger, returning Grim Initiate from your graveyard to your hand.
Activate Phyrexian Altar by sacrificing the Zombie Army token, adding one mana of any color.
Repeat.
Once you have infinite mana, you may repeat the loop without sacrificing the Zombie Army token.

Infinite colored mana

Infinite death triggers

Infinite creature ETB

Infinite creature LTB

Infinitely large creature

Infinite creature sacrifice triggers

Infinite storm count

Koll, the Forgemaster

Phyrexian Altar

Obsidian Battle-Axe

Grim Initiate

Activate Phyrexian Altar by sacrificing Grim Initiate, adding mana.
When Grim Initiate dies, it triggers, creating a 0/0 creature token with a +1/+1 counter on it as well as a vigilance counter due to Tayam.
Activate [[Tayam]] by paying {3} and removing three counters from creatures you control, milling three cards and returning Grim Initiate from your graveyard to the battlefield with a vigilance counter on it.
Declare that you are activating Tayam's ability again, 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}.
Complete the activation by paying {3} and removing a vigilance counter and a -0/-1 counter from the creature token and a vigilance counter from Grim Initiate.
Holding priority, activate Phyrexian Altar twice by sacrificing Grim Initiate and the creature token, adding mana.
When Grim Initiate dies, it triggers, creating a 0/0 creature token with a +1/+1 counter on it as well as a vigilance counter due to Tayam.
Resolve Tayam's ability, milling three cards and returning Grim Initiate from your graveyard to the battlefield with a vigilance counter on it.
Repeat from step 4.

Infinite self-mill

Wall of Roots

Agatha's Soul Cauldron

Tayam, Luminous Enigma

Combos with "Grim Initiate"