When Infested Thrinax enters the battlefield, it triggers, causing you to create a number of 1/1 Saproling creature tokens when a nontoken creature you control dies.
Activate Shilgengar by sacrificing Brokkos.
When Brokkos dies, Infested Thrinax and Pitiless Plunderer trigger.
Resolve the Infested Thrinax trigger, creating six 1/1 Saproling creature tokens.
Resolve the Pitiless Plunderer trigger, creating a Treasure token.
Resolve the Shilgengar ability
Activate Shilgengar four times by sacrificing four Saprolings.
When the Saprolings die, Pitiless Plunderer triggers four times, creating four Treasure tokens.
Resolve the Shilgengar abilities
Activate five Treasures by tapping and sacrificing them, adding 2U/B.
Cast Brokkos from your graveyard for its mutate cost by paying 2U/B, putting it on top of a Saproling.
Repeat from step 3.
This intricate loop turns the cycle of life and death into an engine for endless resources, powered by the synergy between your graveyard and battlefield. Shilgengar, Sire of Famine acts as the catalyst by sacrificing your creatures to forge Blood tokens, while Pitiless Plunderer quietly funds your operations by dropping a Treasure token every time a creature perishes. Meanwhile, Infested Thrinax ensures that losing a heavy hitter like Brokkos, Apex of Forever pays off in a swarm of Saproling tokens that feed right back into Shilgengar's altar. Because Brokkos can repeatedly crawl out of your graveyard using its mutate ability, you can fuse it onto a Saproling to repeat the entire sacrificial cycle endlessly—generating infinite mana, tokens, and death triggers to overwhelm the table.