Activate Altar of Dementia by sacrificing Tenacious Dead.
When Tenacious Dead dies, it triggers itself, Pitiless Plunderer and Pawn of Ulamog.
Resolve the Pitiless Plunderer trigger, creating a Treasure token.
Resolve the Pawn of Ulamog trigger, creating a 0/1 Eldrazi Spawn creature token.
Resolve the Altar of Dementia ability, causing target player to mill one card.
Activate the Treasure by tapping and sacrificing it, adding ({B} magic symbol) .
Activate the Eldrazi Spawn by sacrificing it, adding ({C} magic symbol) .
When the Eldrazi Spawn dies, Pitiless Plunderer triggers, creating a Treasure token.
Resolve Tenacious Dead's ability from step 2, paying ({1} magic symbol) ({B} magic symbol) to return it from your graveyard to the battlefield tapped.
Repeat any number of times.
Once you have infinite Treasure tokens, you may repeat this loop spending mana from Treasures instead of Eldrazi Spawns in step 7, giving you infinite Eldrazi Spawn tokens.