I'm going to tell you an experience I had with a certain Skyrim mod, "af7c.esm". It all started when I was browsing an old thread, and the OP was playing through Oblivion with every single mod he could reasonably get on the Oblivion Nexus. Yes, including the ones that were tentacle-rape mods. I hadn't touched Skyrim in a while, so I decided to do the same. I went through the Nexus's "latest files" section and I got the first 10 that were there. One caught my attention though, and it was af7c.esm.
I don't remember much of its description, but it said something about dragons but was really vague. I got all of them into a mod loader, but it would crash to desktop. Fantastic, so I had to weed out the one that was stopping the game. As it turns out, most of them were incompatible with each other, so I decided on just trying out the af7c file. As it turns out, it was incompatible with Hearthfire. And Dawnguard. And even the Update.esm file. I had to disable everything but Skyrim itself.
But it finally ran, and everything was unchanged. I thought that I had ended up with a dud, but after I started up the game I was committed at least play something before I went to bed. Luckily, I was able to load up my old vanilla Skyrim save, and I finished up some quests that I never got around to. But then, I found a dragon. Knowing that I had killed off all the dragons in the area, I knew that this new dragon was made by the mod.
It was called a "Daedric Dragon". The title was a bit of a misnomer, as I was able to kill it as easily as any other dragon so far. After collecting the loot off of it, I sold it in the local town, and was on my way to my original destination. But then the same dragon was in the same spot. I figured I could at least make some decent coin off his loot, so I killed him again. And again he respawned in the same area, so again I killed him.
But on the third time he died his corpse faded away and the Adoring Fan walked out of it. His eyes were inside out (best way to describe it) and were protruding out of his face a bit. He told me of a curse that I had "brought upon Cyrodill" and I got 3 dialogue options.
They all said, "What is to be my future."
He answered, "You must to the Cyrodiil Arena and defeat the 7 challenges to undo your actions."
The dialogue menu popped up but there were no options. Once I clicked the menu the game froze for about half a minute and the Adoring Fan was gone. A marker was placed far off the map to the south, but I decided to go as close to it as I could.
To my surprise there was a valley opened up next to Riften, which allowed passage to Cryodiil, supposedly. The problem was that halfway through there was a giant black wall, where it seemed Skyrim ended. Upon entering, the game froze for almost a minute. I nearly turned off my computer but then the game resumed, and I could see Oblivion.
The only explanation that I can rationalize from this is that the game loaded up Oblivion's map. The game appeared to be running in Skyrim's engine, but I'm just going off of what it "felt" like at the time. Textures seemed to be the same, but all were very low quality. I'm guessing that it did that due to the lowest settings were the default/backup configuration. Ambiance music didn't seem to play most of the time, when it did it was only in a cave or a dungeon (which I soon found them to be completely empty so I didn't bother with them).
Anyways, I traveled down to Bruma, the first town that was closest to where I came from. The map marker was still set on the Imperial City arena, but I was too curious to try and see what the NPC were like, as I saw no monsters or animals outdoors. People were there, but they were stuck in their default T-pose. The NPCs that you could normally talk to brought up the dialogue box, but it was empty and clicking just seemed to close it.
I still felt like I wanted to check out the rest of Cyrodiil, I decided to leave the Bruma and travel to the other towns. It was all the same, people stuck in those lifeless T-poses. This took a few hours to do, as quicktravel didn't seem to work. Horses, while looked normal at first, could still be mounted, but then my character was T-posed on the horse. I quickly dismounted, and decided that walking would at least allow me to be more keen to seeing someone that I could talk to.
And I did! Too bad it was a highwayman. I was startled by his "Pay up, or else." demand, I hadn't heard someone speak for what had to have been hours. I figured that the only way to talk to him more was to not make him want to kill me. So I gave him the 100 gold he requested, but my heart sank when the dialogue box was again empty and he was T-posed. I tried attacking him, but it did nothing as he took the hits and "died" (his health dropped to zero and I could loot his gear, but he was still in his pose).
With all options exhausted, I chose to stop delaying the inevitable and went to the Imperial City. Same as the other towns, lifeless bodies guarded the gate and filled the streets. with the doors unlocked, I went into the arena and there was nothing at first. I waited for a bit, then a dialogue box appeared. "1". I was trying to think if the game broke as my clicks to dismiss the message were in vain, but it went away on its own. A bandit charged in from the gates with a dagger in his hand.
He was silent, but it was obvious he didn't have good intentions. Even if I hadn't been using a character that I had ran through the main quest with already, they would have been easy. After I was done another message popped up: "2". Four Imperial guards ran in seeking my blood. Normally, thanks to the level scaling that skyrim has, they would have been very difficult to take on at any level. But as this seemed to have not worked, I was able to finish them off without having to resort to using my potions. "3" was a different story, there was no way I would have handled the horde of imps that it sent after me without a potion to heal up in between their barrage of fireballs.
This battle continued on, by "5" I ran out of any healing items. At "6" I had created over 10 save files dedicated to different parts of a battle I was in; if I had saved an unwinnable scenario, I would try to trace back a few saves until I could actually stand a chance.
It all came down to "7". Even though I had faced all enemies and monsters from the Oblivion game from this point on, I found that the final challenge was a familiar foe. It was the Daedric Dragon, but I wouldn't have known if the game didn't label his character that. I was grateful that I saved immediately after the dialogue box closed, because it didn't even take a second for the dragon to obliterate me. He was moving at least 3 times faster than the last time we fought, and the fire he breathed would instantly kill me before I could tell that he was charging it up.
I tried to fight as well as I could, and I was able to get his health down to 50%. But when I did, he flown off, and began to launch fireballs that were almost as large as the fighting area at me. Tiredness got the better of me, and I decided to pause and call it a night. When I woke up, my computer was showing my desktop. I tried to launch the game with the .esm, but it crashed to desktop again. I checked, and the only .esm file that was launching with it was Skyrim.
And that's my story. Sorry to have it end anticlimactically, and it's unfortunate that I couldn't see it till the end. But, the way it worked was so unreal, that I can't even fathom how it worked. I still wonder if I just been a dream, and it never worked to begin with.