TL;DR
New Oblivion Realm dungeon. Hooked into the vanilla pool for Oblivion gates. One .esp, no new assets, highly compatible. More-or-less adheres to vanilla design principles, so should mesh seamlessly with the game/any install.
DESCRIPTION
If you've been playing Oblivion for a while, you probably know that there's an extremely limited pool of Oblivion Realm dungeons for the game to draw from for its nearly 60 semi-randomly placed Oblivion Gates in the game. Specifically, there's seven. If we add that up with the number of unique Oblivion Realms--which are largely used for setpiece main quest battles and, for some reason, Cheydinhal--the number rises to 11.
Anyways, this basically means that there's fewer bespoke Oblivion Realm dungeons than there are cities in the game. However, each city gets a guaranteed Oblivion gate spawned outside of it (it doesn't help matters that the Imperial City can receive three). There's a main quest, too, which optionally requires you to close all Oblivion Gates spawned outside of cities. Needless to say, Oblivion Realms can start to feel more than a little repetitive if you do this, or just want to hunt Sigil Stones for their powerful enchantments.
For ages, I have wanted a mod to remedy this repetition and expand the Oblivion Realm dungeons. Perplexingly, nobody has made one yet (seriously, it's weird--the tutorial I used to learn how to make dungeons and worldspaces literally teaches you by making an Oblivion Realm dungeon).
Enter this mod! Which... does not fix the problem. But it wanted to.
This mod adds one--just one--brand-new, fully navmeshed Oblivion Dungeon to the game. It only took us almost 20 years to get to this point.
If you can't tell from my lengthy pre-amble, the intention was bump the number of Oblivion Realm dungeons up to match the number that spawn outside cities (so I was planning on adding about five more, so that you'd at least see a unique dungeon if you mainlined the main quest + closed all city Gates).
Well, I am but a lowly animator. Making dungeons is really hard. My ambitions to nearly double the amount of Oblivion dungeons were quashed by the Oblivion Remaster's release, being new to the process, general burnout, and life itself. But I did finish one dungeon. With some prodding from some friends, I've decided to release it. It'll do more good on the Nexus for the 100 or so of you who still play this game, and hopefully you get some joy out if.
Will this mod expand? Look, no. This one won't. If I do end up making more dungeons, I'll probably release it as a new mod. I'm releasing this as a single mod because it's unlikely I'll muster the motivation to do that, though, but you never know. There are some edits to ToddTestCell, where I set up doors that teleported me directly to the dungeon I was working on. If you wanted to go into Todd's house and check out those doors, you can see what I was working on before I lost steam (one of them is a lot further along than the other). Maybe it'll inspire some of you. Maybe you being inspired will re-inspire me, and we can work together and expand the dungeons. Anything is possible.
Oh, also, the test cell edits constitute the only area where the mod could "conflict" with something, but I left them in because a) you've no reason or even ability to go there in a normal playthrough, and b) because it's far more worth it, I think, for y'all to be able to poke your heads in on the unfinished stuff I left behind and see if you want to pick up the torch.
Anyways, have fun!
THE DUNGEON
The intention here was to hew as close as possible to the vanilla design, while still keeping things fresh and exciting. After spending some time zipping around the vanilla Oblivion dungeons, I realized that they're fairly diverse. The tileset is painfully limited, but each gate generally has its own theme, or vibe, or design principle that it's centered around (at least as far as I can tell as an extreme amateur).
As this was the first dungeon that I started, I decided to make it a bit smaller than usual, but denser. Mostly, this means a couple of branching paths in the overworld. Oblivion isn't crazy about doing this (Bethesda isn't in general), but I think I mitigated it by always having the Sigil Tower visible. For the most part, it's pretty clear where you're supposed to go, and if you do happen to take a wrong turn, the map is small enough to where it's not a pain to turn yourself back around. Plus, if you have good Acrobatics, you can probably skip between paths pretty easily. To that end, I'd recommend installing a mod that bumps up the damage of lava, or just adjusting the values to your liking in something like xEdit but, hey, it's your game.
To support this bit of design, I decided to lean into what little I know about the "lore" of Elder Scrolls. Mehrunes Dagon is the "Prince of Destruction," as the game calls him, but he's also the Prince of Change. So, to support my smaller map idea with the branching paths, I made the world look it was in the process of dissolving. My aim was to create a place that looked like it was once a mighty fortress that has been slowly consumed by the influence of Mehrunes Dagon, slowly destroying itself until it forms something new. The chambers in the tower have been named to be consistent with vanilla, but also to suggest a process of rejuvenation through the destruction/change.
Anyways, at the end of the day, it's a dungeon for a nearly 20-year-old video game. Hopefully it's fun. The very few branching paths can lead you to some goodies, but there's only one boss chest (in the Sigil Tower), as usual. I tried to make enemy placement fairly reasonable, and I placed a tasteful amount of blood fountains and magicka fountains around the map after stress points. I confess that, outside of the lower halls of the main tower, I did copy-and-paste a lot of interiors and simply tweak them--but the vanilla game does this, too.
My hope is that this dungeon more-or-less fits in seamlessly with the vanilla game, although I'm sure it'll stick out a little bit by virtue of it having a unique design (again, though, this is consistent with how vanilla handles Oblivion Realm dungeons) and by it being new (the game is old).
INSTALLATION
It's only one .esp. Download it, install it, check the box, and you're good to go. This mod uses all vanilla assets, baby.
COMPATIBILITY
Should be compatible with everything, since there's literally zero other mods that attempt this. The mod draws from vanilla leveled lists and loot forms, so as long as your overhaul mod also uses/edits those, you're fine (I personally use Ascension, and it works seamlessly with this mod). If your overhaul adds new leveled lists, you'll likely need a consistency patch with this mod. I'm afraid I won't be providing them myself.
Instructions
Install with mod manager or manually (drag and drop).
Additional Oblivion Realm
Install via mod manager. If you're a weirdo manual install, drop in your Data Files folder and tick the box.
Additional Oblivion Realm - Door Offset Fix
Use this OR the Main File, not both. Additionally, only use if you notice that the animated doors (not loading doors) are offset in the Citadel/Sigil Tower. I'm looking into what causes this. I think a mesh edit from UOP offsets the door.