Hey DMs, how are you guys/girls doing? Looking good today. That shirt (or the absence of one) looks good on you!

Aaaanyways.

So, I've been running this campaign for a long time now, some months have elapsed and since day one I've been talking about this really evil aura that started just around a creepily abandoned cathedral but then engulfed the mountains around it and then a city. After months, half of the map was covered in this darkness with kings and queens begging people not to go in there and so finally the group decided: we need to stop this.

Their first contact with the effects of the darkness was a female drow who was a potion seller. She looked normal, except for her veins which appeared to be popping out in a blueish color. With time, more and more people started appearing with blue veins. Later some extra creepy monstrosities came that even the most experienced players had never seen, they basically had normal monsters stats but I changed their appearance to be humanoids and VERY creepy. So the group found out that these monsters were those people with blue veins in a more advanced state of the "disease". So it made sense that the kings and queens didn't want people going into the darkness. You will get this weird disease if you go there and later turn into a monster.

We started off slowly, one wise NPC that they trust said: "You wanna go in there, you need to prepare." and so they heard of this magic item which could delay the effects or even stop it if the player were lucky enough. So now it was a matter of "just don't stay inside the aura too long". Well, now the stage is set and we have nowhere to run anymore because all the players are looking at each other like "if we don't stop it now, it won't stop". They know where to go, they know who to stop, my problem is the way there.

I want your help in creating this nightmarish place where the players will never want to set foot again. I already thought of the idea of "this is much more dangerous than outside, so they are going to have encounters like crazy" but that won't work. It will just be repetitive or even boring. I've set some big and scary monsters to be avoided at best or if they were to find one, run away as fast as they can. They are informed of those already. I'm also thinking of adding some shadow people, which would be harmless but could have some purpose or effect. Like they watch the players from a safe distance much like spies for the enemy and if there are enough of them, they could attract something big and awful.

What do you guys think? Help me out here with some awesome idea!

Are you familiar with the movie/book Annihilation?

Anyway, Matt Mercer has a set of optional corruption rules on DM's Guild. It's a free download and it feel like something you could work into your area of darkness.

Certain landscapes house deeply evil places or artifacts that corrupt the very nature of people who spend time around them. Sites where the presence of such terrible wickedness can slowly twist and infect the physical forms of even the most holy of creatures. Unhallowed or cursed locations that erode the drive and will of those that linger. When sleep comes to those that stay, it's this period of vulnerability that invites the darkness in. This ruleset, heavily inspired by and condensing many elements of the Heroes of Horror Taint rules, allows you to have this dark influence slowly infect your adventuring party, helping build tension as they traverse the shadowed lands.