shin megami tensei

Shin Megami Tensei or Megami Tensei (真・女神転生, lit. "True Reincarnation of the Goddess") is a franchise of JRPGs developed by Atlus, originally based off the "Digital Devil Story: Megami Tensei" series of books.
The Shin Megami Tensei games is one of the most popular branches of the franchise, with most games being set on a post-apocalyptic setting, while having many fantasy elements and featuring demons (based off cultural/religious/mythological figures) as both enemies or allies that you can contract and fuse into stronger demons.
Out of all the spin-offs the franchise would have, one of them would eventually develop into the Persona series, which would exceed the rest of the Megami Tensei franchise in both sales and popularity.