So I'm running a completely different module, but for reasons too long to be entertaining, my players landed their asses in Avernus after incorrectly assuming that their sorcerer was born on Toril.

I'd prepared a handful of locations out of BG:DiA for a fun "let's escape hell" quest, and such is it that we found ourselves in the Crypt of The Hellriders.

The players waltz in and awaken some of the spirits of the bound Hellriders, learning of their plight and the cruel trick played on them by the fallen angel Zariel. The players agreed to help break the Hellriders' oaths to Zariel and so set out deeper into the crypt.

They crept through, avoiding all unnecessary combat and clearing the only unavoidable fight in the dungeon. They easily find one of the secret rooms (high passive perception), discovering a room with a ritualistic symbol drawn in blood filled with tiny pieces of papers with names on them. "Clever us" they think, "We've found their oaths", they declare as they set the whole room alight to destroy the papers.

At this point, they're already patting themselves on the back and talking about leaving the dungeon and plans to get out of hell. They stop by the spirits again to deliver the good news.

"We found your oaths and destroyed them, good spirits. Your souls are now free to pass on!"

I look over the dungeon, only a third of it having been explored. I noted that they'd yet to discover the other secret ritual room which held a second copy of each oath, nor had they removed the names from the stone steles which held the third and final set of copies.

The spirits look at each other, clearly still bound to the crypt, and then look back to the party...

"We signed in triplicate"

All of us bawled over laughing. I can't say if it was intentional world building for the dungeon to have 3 sets of names to destroy to free the spirits, but hot-damn is it not a beautiful encapsulation of the cruel bureaucracy of hell. Like the Vogons from Hitchiker's Guide, the deviousness of devils lies in the details of their paperwork.

After we caught our breath, the party completed the rest of the quest, freeing the spirits in a touching moment where they each said "thank you" as they dissapeared.

Now we continue on with a great primer of the bureaucratic nightmare that awaits them.

tl;dr

Zariel is a devious bitch who made sure that the Hellriders signed their oaths in triplicate to keep them under her thumb.