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 Yahenni 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 Yahenni ability.
Activate Yahenni four times by sacrificing four Saprolings.
When the Saprolings die, Pitiless Plunderer triggers four times, creating four Treasure tokens.
Resolve the Yahenni abilities.
Activate five Treasures by tapping and sacrificing them, adding ({2} magic symbol) ({UB} magic symbol) ({G} magic symbol) ({G} magic symbol) .
Cast Brokkos from your graveyard for its mutate cost by paying ({2} magic symbol) ({UB} magic symbol) ({G} magic symbol) ({G} magic symbol) , putting it on top of a Saproling.