I stumbled on a nest of vampires. I could take out the fledglings with a single double-fisted firebolt, and the full vampires took a bit of more than that, but I was feeling pretty confident anyway. And then I ran into the local master vampire, who had a half-dozen regular vampires with him.
I died a lot to that guy.
After the nth reload, I began to look in my inventory for something that could help. I remembered that 500 gp Scroll of Undead's Bane (or whatever it was called) I found in my first dungeon. Vampires didn't count as undead in Oblivion... but I had to try. I figured it might hurt them enough that I could finish off a few of them with fire spells before they healed back up by absorbing my health, and then maybe I'd have a shot at dueling the master.
I panicked for a moment as the scroll had several seconds' casting time, during which my health dropped dangerously low and the master vampire somehow cast a spell that unequipped my clothes. Then a blue wave of energy burst forth, instantly vaporizing all the regular vampires into smoking dust piles, and reducing the master to a whimpering lump with a sliver of health. "Mercy!", he cried.
Magic is awesome.