Whether humanity directly thinks about it or not, death- and by extension the underworld, are practically the fulcrum of the higher mythological order.
This wasn't originally by design, but over the ages humans started thinking about death more and more.
Some feared it, others begged for it.
One person mourns deeply the loss of a cherished family member and wishes for them to see only the most blissful of rests.
Others celebrated the death of adversaries and prayed that they wound up in the foulest hell as they urinated on their graves.
All of those thoughts, wishes, fears, or sentiments can all be converted into prayer for the death gods- fueling the dead kingdoms that they rule and making them quite formidable.
It is perhaps the reason gods of death are so widely disliked in the heavens associated with their various pantheons.
