For me, it was when I was running around Blackreach, and came over a hill to see a veritable army of Wistmothers/Shades/Whisps. I couldn't count them all, and didn't get close enough to see if it was just one Mama that summoned the rest. I was running the Ice Storm/Flaming Familiar combo, half of which they're all immune to, so I hunkered down behind some cover, and sent my little fireball on legs to thin them out. I got a lot of Conjuration experience, but they just kept making more, so eventually, I kited them over to Falmertown, then went to find some reinforcements. A wandering giant, and 2 Steam Centurions helped a lot, so eventually, I could mop up, and go onto Mzark. Took the better part of an hour, the whole time white knuckle, and borderline panic attack. I survived, though, and was glad for the experience afterwards. Times like those are what I play for.