Citadel of Madness is mod for The Elder Scrolls 4: Oblivion that’s over a decade old. (It was first uploaded to TESAll in 2013, and significantly updated in 2022 with a 2.0 release.) It adds a new landmass above the Shivering Isles, a floating archipelago where hundreds of new NPCs live and where you’ll find a new questline the mod’s description calls “comparable in duration to any Cyrodiil guild.”
Citadel of Madness was created by modder Lariatius and translated into English by Vorians, who has adapted a bunch of other Russian mods as well. It contains 38 quests, 15 of which are sidequests, and promises more than 25 hours of playtime. There are over 3,000 lines of dialogue (no English voice-acting, however), and a variety of new alchemical ingredients and concoctions, new dungeons, and new enemies—including bosses.
Given the connection to the Shivering Isles, you can expect Citadel of Madness to be quirky, to say the least—several of the new NPCs look like they stepped right out of the Monster Factory. You should be at least level 18 to play the Citadel of Madness questline, which is begun by traveling to a watch tower you’ll find north of New Sheoth in the Isles.
To install this mod, you’ll need to download the files from Nexus Mods. They add up to almost 900 MB, which is less than some Skyrim mods but definitely on the chonky side for Oblivion. You’ll need to own the Shivering Isles expansion as well, but you probably already do since it’s packaged with all digital versions of the game today because of how amazing it is.