The Raven Queen

When a creature dies on any of the three material planes, their soul enters Sheol. From there they are guided by Magda, The Raven Queen's daughter to Sigil, the Heart of the Cosmos.

When a creature comes to Sigil, they are presented at the Grand Auction, where representatives of each of the sixteen outer planes bid on the souls of the newly arrived.

Economy of the Cosmos
These souls are bought with pledges, the currency of the cosmos. Pledges can be used to purchase souls for an outer plane or to transport one from an outer plane to a material plane. The more powerful the transported creature, the more pledges it takes to move.