Saṃsāra

Related:
* Ghosts - commonly problems with the cycle of rebirth causes surfacing of numerous spirits
* Past-life memories

This is unlikely to be anything more than a plot element, though it isn't impossible for this to be actually part of gameplay or a location.

Named Saṃsāra here because there's no English word for the concept, though some terms are used variously. Reincarnation itself is unusable since it can occur outside of the existence of the cycle, nor does the cycle necessary have any traits of reincarnation (nothing carried over from previous life).