When Prosperous Innkeeper dies, Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return Prosperous Innkeeper from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Prosperous Innkeeper enters the battlefield, it triggers, creating a Treasure artifact token.
Activate the Treasure by tapping and sacrificing it, adding ({2} magic symbol) .