Since the 4.5 release the mod deals with the official expansion too. Every part of the mod now affects SI with the same approach vanilla Oblivion is dealt with. This doesn't however mean that SI is required, if you don't have the official expansion you can install the non SI version of the mod. Apart from expanding the mod to work in the new lands the SI version has some new unique features too like the Amber/Madness Items Forging System Overhaul (check the Leveled Quests section in the readme for more info). A lev 9-12 char at least is now recommneded to adventure in the Shivering Isles.
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Tired that nearly all creatures level up with you? Bored that at level 25 all bandits of the world are god-like ones with daedric equipment? Than this mod is for you.
I think that the game has to be challenging but that not every fight has to be an epic one. This mod aims to create a more realistic and enjoyable experience, it will not make the game easier, just more credible (actually it can make it harder depending on the parts you decide to install)

This mod changes nearly all creatures lists to provide a more static, non leveled, experience. At higher levs you will still find very strong stuff but you will often (for example) find normal skeletons and zombies in dungeons, common wolves in the wilderness and so on.. not every NPC will level up with you... some of them will have level caps.. others will be completely static... on the other hand.. at lower levs life will be much harder as, depending on where you will adventure, you will be able to meet high level enemies. Items lists were changed with the same goal, in vanilla Oblivion at say level 20 you used to find only high level armors/weapons and low-mid stuff nearly disappeared from the game, now it will be much harder to find high lev equipment, and low-mid lev one will still be common at high levs (you can also decide between 3 loot systems: normal/hardcore/extra-hardcore)

One of the strengths of the mod is it's modular nature.. every part of the mod is optional and you are able to customize it to your taste during the installation. Stunning in-game customizanbility is also provided giving the ability to adjust nearly every aspect of the mod.

The optional parts cover many things like guards (all guards used to keep leveling up with you, so no matter how strong you were they were always a bit stronger than you. Using this part of the mod, while some guards still level up with you, many of them will stop at about levels 15-25. Stronger guards will usually defend more important places/people), vendors (you used to find chainmail/dwarven mithril/orcish armors and weapons in creatures/npcs loot before seeing their appearance in shops, this made armor/weapons vendors nearly useless after reaching level 6-7, now you will find those stuff in shops many levels before but at increased prices), new unique enemies/bosses (a small chance to find stronger unique bosses with names and personalized combat styles/loot), more enemies (more creatures/npcs in dungeons, aviable in three variants) houses/dungeons chests (improved, more interesting, diverse and often more rewarding but still balanced loot), quests (tweaked main and guilds quests to make late quests harder for unexperienced characters, forcing to level up so you can't become master of the fighters guild at lev 8), arena (later ranks will be much harder for weak chars so you can't win it with a low lev character) and much much more (read the full readme for a complete list with detailed info).

There's also an items add-on adding 1.400+ new (carefully balanced) quality items to loot lists and a creatures add-on adding 120+ quality user made creatures of many types/level ranges and dozens of new retextured variants of existing creatures/aniamls.. unmatched variety in all circumstances! This part of the mod is aviable in a few variants.. you can install it with/without size rescaling scripts and with/without additional Ai/Behavior scripts to make creatures/NPCs even more varied and realistic.