When Memnite dies, Sigil of the New Dawn and Pitiless Plunderer trigger.
Resolve the Pitiless Plunderer trigger, creating a Treasure token.
Activate the Treasure by tapping and sacrificing it, adding ({W} magic symbol) .
Resolve the Sigil of the New Dawn trigger, causing you to pay ({1} magic symbol) ({W} magic symbol) to return Memnite to your hand from your graveyard.