After you run back and forth fetching all the items for that quest, then close the daedric boxes in the towers, your sent to the bottom of the waterfall to use the scroll to see the daedra. Spider daedra spawns... mid fight she vanishes. Can not re-summon. Have to abandon and did the whole thing again. Vanished again. Ended up having to wait for someone to show up so it could be burned down in a matter of seconds before it de-spawned again.
I filed a bug report, but posting here for other people to be aware.
"There is no moisture in your angry stares." - Laughs-at-All
"I don't know why I bother guarding you horrible people." - Orama Sadas
"Scales here is about to have a really bad day..." - Valeric
"Just tell me what you're doing here before I turn your heart into a tomato..." - Sereyne
"Break those rocks! Dig those ditches! Why??? Because I want you to!!!" - Ifriz the Unraveller
"There are worse masters than I. Far worse." - Molag Bal
"I humiliated the Daedra in Mehrunes Spite." - You, when turning in a specific Undaunted Daily.
"I'm not finding you very pleasant!" - Adla the Brewer
"Old Ri'hirr likes his birds slow and stupid!" - Old Ri'hirr
"When things get dirty... Oh, I get so flustered." - Meredil the Archivist
"Too many Argonians about these days..." - Davon's Watch Guard (though I think this one has been removed from game)