Reaper is the world where powerful beings are created by Kaguya Otsutsuki herself creates to make her army. She successfully made a complete being known as Nichan who she then snuck out with Black Zetsu outside of her portal and sent to Earth.
Reaper is a very dark and evil place where chakra begins to seep from those not created or born there. Whenever a being dies, their soul is trapped there for eternity.
Within the world lies dark dragons and trapped souls. Zetsu soldiers and Kaguya's Reaper army can also be seen there. Kaguya's aspect was originally born there but was sent to earth just like Black Zetsu and an army of White Zetsu.