I must've done my character right because I didn't experience much in the way of difficulty until getting up around level 18. One that stands out as being particularly difficult though, is Umbra. I was low level, don't recall how low, but low. I initially learned of Umbra the sword from the Elder Scrolls novel The Infernal City, so I was excited to learn of Umbra the person and sword being present in Oblivion. I immediately sought the warrior out. After several failed attempts, I was finally able to overcome Umbra by luring her to a narrow hallway and slashing from safety behind my conjured frost atronach. Several frost atronachs were sacrificed for my success. Face to face with Umbra, I was no match- not even close. I wonder how the 20th level version of my character would've fared... Other difficult encounters include trolls, which are so much faster than their Skyrim contemporaries, especially in numbers. And particularly troublesome for mages such as myself, (while not a pure mage, most of my offense and healing abilities are magical in nature), are higher level undead casters like Wraiths and Liches. The sheer variety of magic these enemies can bring to combat is impressive. They can hurt you in so many ways, including rooting you in place by draining your strength, and leaving you unable to cast a single spell by using silence spells on you. If you don't kill them quickly or get away, they'll render you useless. Things like Minotaurs, Ogres, and Daedroths are tough, but with my spells they're more nuisance than anything. Storm Atronachs are a bit of a hassle... which reminds me that I need to acquire that conjuration spell. Some of the high level pure mages and necromancers can be very dangerous, but they're usually so weak to melee weapons I can just conjure a distraction, then charge in and hack away. What in your experience have been the most difficult and troublesome foes in Cyrodiil?