S02E17 - The Vast Emptiness of Grace - Descent Into Avernus - Oneshot part 2

Tier 3 Adventurers League: One shot
A beleaguered city overrun by enemies, a ruin recently returned to the world via a massive planar shift.  A group of powerful adventurers with a celestial benefactor has been tasked with entering the city in search of an ancient relic of divine Ilmater - an ancient artifact once belonging to Ilmater
himself. Beneath the holy city of Elturel lay a reliquary—the Dolorous Adytum. What
dark secrets lay within? It’s time to find out.

The group has spent time together and has forged an tight alliance tested on the plane of Avernus and the fires of hell itself.  Having successfully contributed to the rescue of thousands of innocent souls, attention is now directed to the city in ruin.

Party alignment is with the holy triumvirate:
Ilmater - god of endurance.
Torm - the True
Try - of justice

Many years ago the city of Elturel was inundated and on the brink of collapse due to swarms of undead. The Archangel Zariel arrived, answering the prayers of the faithful and she bestowed a companion above the city, a beacon of light that burned all undead.
Recently the city was transported to Avernus, the 1st layer of hell, now ruled by the arch Devil Zariel (yes one and the same).  The city has been rescued, returned and restored but now without the companion, looters, devil remnants and worse roam the city ruin.  A group of monks taking a vow of silence and solitude had a temple - an Adytum - deep below the city - and they guarded the relic mentioned above.  Undead survived the companion by digging deep beneath and biding their time.  With the companion gone they are swarming to the surface again (mostly at night.... mostly).
Now it is night.  While the undead below have left their sanctums it will be less difficult to get below ground.  Advice for fliers is to stay below rooftops and out of line of easy sight.  They initially sent griffon riders in - which were promptly swarmed by wraiths.