The afterlife is a huge plane, not flat.
There are plateaus
The plateaus are as large as needed; each is a realm protected by the gods of the past and of the now.
Power is that of will, boosted by the faith of the followers
The vast valleys between the plateaus are filled with the faithless dead
, the faithless dead die again and again, lacking the divine protection of the realms of the plateaus
And there is hunger
that can cause one to consume the bodies of the dead souls. This can nourish as well as corrupt. The act of nourishing happens when healing your fellow dead. Corruption from consuming.
Your power can be increased through the faith of your followers. But they cannot be told this in the beginning. Only voluntary faith creates the connection between the faithful and the bright one.
Bright one must protect.
can start raising a plateau.
The dark ones feed and warp those they consume into twisted parodies of the souls they were in life.