Necropolis

Eight men, three after being killed, and five after entering a portal, acknowledge the life they knew ended, but continues within the underworld.
There they must face the Shetani, misanthropic and supernatural forces personified as men, who work in concert with the necromancer, a chief practitioner of black magic and sorcery. The man eating ghouls they encounter while on their never ending quest to find safety aren't any more hospitable.
In the midst of desperation to discover whether they can find some place of refuge or save their souls, five of the eight men learn the necropolis, a diamond-studded jewel imbued with a magical power, is critical to their having any hope of finding salvation.

This book is the introduction, the first eight chapters of a longer work on the series.