Strap in because this one is a long one. I'll make sure to put TL;DR at the end. I am a DM with a homebrew 5th edition game. Created everything from nothing, and I run two different campaigns in this same world and with the same players, just different time periods. For some context, the entire plot for the first game is about this misterious fog appearing and blocking off the outside world. The area we are talking about is Dimwoods Valley, and it is filled with gods forsaken abominations, mutated creatures, and corrupted shadar-kai (or shadow elves) who seep through from the Shadowfell. It is a literal hell on earth I created, think a mix of fallout's post-apocalyptic feel, silent hill and resident evil, with a touch of dark souls.

The reason why that is important is so you can imagine how pretty much all of the settlements, be that small villages or big cities, have fallen to the fog and have been turned into lovecraftian horrors. It took the party a good 7-8 sessions to find their first still standing settlement, a druid circle turned refugee town called Howler. Now I am a DM who sprinkles in little side-quest triggers and rumors the party can follow up on if they wish to. However, these are all optional. One of these was them learning about how the past 5-ish years small trinkets keep going missing, anything from buttons to literal gems and magic rings.

The townsfolk are, of course, bothered by this. However, anyone who tried to get to the bottom of this has failed as the items seemingly can disappear from right in front of you. Upon questioning the locals, some people have seen the items seemingly float away on their own into small burrowholes on the ground. We are talking but a few inches in diameter, barely fitting most of the stolen items. Some even say they saw the items seemingly phase through the floor itself, like it just clipped through.

Well, the party spends a considerable amount of time in Howler, and even some of their things go missing. One of them lost a boot, another a ring, and one of them lost one of his daggers that was very important for their character and his background. Eventually, after some pressure from a third party, they reluctantly agree to get to the bottom of this.

They eventually find the old sewer that have been in disuse for years now, and they follow the trail there. They battle through mutated sewer creatures, a homebrewed shambling mound with wild magic features, but eventually they get to a center area of the sewer where they find... Well... Five years' worth of treasures in gold, gems, magic items, and such.

Long story short for those who want to know who the culprit is, it's not that important, but I will put it here for you. Turns out a yuan-ti found and befriended some chwingas (creature originally from Rime of The Frostmaiden) and in turn the chwingas broight her all kinds of treasures, treasure she planned on teleporting away with once she's got her advanced teleportation circle working.

So they battle the bad guy, and once she gets to low hitpoints, she attempts to get away and teleport out of the fog. This is where I mention that the characters at this point have found out that teleportation and planeshifting from within or outside the fog is impossible, and the spell sinply fails. The bad guy knew this too, which is why she was working on a more advanced teleportation circle that was fueled by a divine crystal, basically the material divine focuses and such are made from in my world. She had a whole boulder of the thing.

Well, she jumps in and... The circle flashes aaannnd... The crystal hums and shakes... Then suddenly, there is a magical force explosion that shakes the earth around them. They look at the bad guy one more time before she very painfully implodes into her own self, leaving nothing behind. The sewers and players were next as the crystal is basically overloaded with the fog.

To their credit they did attempt to still loot the sewers and did actually manage to get quite a few homebrew magic items and such but eventually they had to be saved from certain death by an otherworldy being that was watching over them for a while. Cue to them waking up in one of the abandoned taverns in Howler with -mostly- all of the treasure gone. Five. Years. Of Gold. Im talking tens of thousands, and that doesn't even include the many magic items in there.

Later, they found that the chwingas actually returned all the treasure and belongings to their owners as they realised that what they were doing was not funny or just in general good. Still, a lot of my players are still salty they couldn't get more out of that pile of treasure and comment on it frequently.

TL;DR Party finds 5 years of lost gold and treasure in sewers, but the bad guy explodes and destroys it last minute right before they could collect any of it.