Once you grovelled through every hick town mage guild in Cyrodil and got admitted to the academy, you could craft custom spells; including the normally-unhelpful "make target invisible" effect.
Probably not the intended solution; but beat the hell out of doing it "correctly".
More generally, I'm off two minds about Oblivion: the Ye Olde Magik Middle Ages setting was...not impressive...after Morrowind; but the Thieves' guild and Dark Brotherhood questlines were arguably substantially better than the ones in Skyrim(Dark Brotherhood just had way more good bits and nasty twists; thieves' guild actually involved a grand heist rather than mostly heavy dungeon crawling with the actual thieving left to tedious radiant quests) and the Shivering Isles were pretty cool.
Morrowind definitely hand the coolest setting; though its...advanced age...is harder to overlook than it once was. Skyrim vastly improved vs. Oblivion on technical merit and average style content per unit mass(and levelling mechanics that weren't utterly broken, that's always a plus); but there were some regressions.
I'll never understand who decided to ruin the empire's Roman style for brownish medieval generica, though. Nobody could possibly have thought that that was a good plan.