Sacrifice Priest of Gix to Altar of Dementia, milling target opponent for 2.
Pitiless Plunderer and Nim Deathmantle triggers; resolve them in that order.
Pitiless Plunderer generates a Treasure token, crack it to float ({B} magic symbol) .
Resolve Nim Deathmantle's triggered ability, paying ({4} magic symbol) to recur Priest of Gix to the battlefield (spending initial ({3} magic symbol) ).
Priest of Gix's ETB trigger adds three ({B} magic symbol) .
Repeat from the top, milling target opponent infinitely.