Cast Dark Ritual by paying ({B} magic symbol) , adding ({B} magic symbol) ({B} magic symbol) ({B} magic symbol) .
Activate Phyrexian Altar by sacrificing Possessed Skaab, adding one mana of any color.
When Possessed Skaab dies, The Cauldron of Eternity triggers, putting Possessed Skaab from your graveyard onto the bottom of your library.
Activate Grenzo by paying ({2} magic symbol) , putting Possessed Skaab from the bottom of your library into your graveyard, and then putting it onto the battlefield.
When Possessed Skaab enters the battlefield, it triggers, returning Dark Ritual from your graveyard to your hand.
Repeat.
Once you have infinite mana, you may activate Grenzo's ability any number of times.
Results
Infinite colored mana.
Infinite death triggers.
Infinite ETB.
Infinite LTB.
Infinite magecraft triggers.
Infinite sacrifice triggers.
Infinite storm count.
Put all creature cards with power less than or equal to Grenzo's from your library onto the battlefield.