This is the mission I prepared for players on my server. They received several clues in discord and had to find the location and win the mission.
The idea was to find the location, find the way to get in, get the quest item and find their way out.
There are several things to do if you want to add this on server for your players. The first thing is to paste this blueprint with infinite health so it cannot be damaged. The second one is to destroy ward and replace it with one that could be destroyed (this is one of goals for the mission). When it is destroyed, players will be able to get quest item and open trap door hidden under the carpet and this will be their way out.
And finally, you will need to make vines and trees yourself because I couldn't copy it with Planbuild for some reason. Add some draugr spawners around so archers can keep your players on move. Also, all items on item stands are level 1. If you want to prepare a prize for your players, add better/upgraded weapons.
This build is copied from my vanilla solo play, so there is a lot of treasure inside. I didn't want to remove it because it looks nice. It may be too much as the prize so make agreement with your players that they don't take anything but quest item.

Required mods (for admin only): Planbuild, infinity_hammer (to copy trees and vines)
Optional mods (for admin): world_edit and spawner_tweaks (for timed spawners around the build).
No mods required for players.

Story for the mission:

Grettir the draugr slayer

While still very young, Grettir kills a person because he thinks they have taken his food bag. Despite attempts to pay compensation, he is temporarily banished from Iceland and sentenced to lesser-outlawry for three years. Before Grettir left Iceland his mother Asdis gave him family heirloom sword Jökulsnautr, which will later yield him many triumphs.
Originally, the ill-tempered Grettir experienced some success. He spent three years in Norway performing many brave deeds. On his return to Iceland he saved the people from the malicious draugr Glam the shepherd (Glámr), who was ravaging the countryside. But Glámr used his last breath to lay a curse on him, what led Grettir in a different direction. As a result of Glámr's curse, Grettir became disastrously unlucky, only grown weaker/never stronger and became afraid of the dark.
Later, on an errand of mercy, Grettir accidentally set fire to a hall in which a chieftain’s son burnt to death and so he was outlawed again.
This outlaw status forces Grettir to live on the edge of society and opened him up to being hunted by others and being betrayed by other outlaws. Grettir lived in various places running from enemies and slaying more monsters.
At last his enemies overwhelm him with the aid of witchcraft. Grettir died in his last stronghold located in the dark and wet place haunted by both his fears and his enemies.
Find his last stronghold and recover his trusted sword Jökulsnautr.