Just finished Azura's Relics. Used scroll and summoned the spider daedra. It only appeared for about 5 seconds though before despawning. Was able to summon it again and then quickly killed it to get the update to the quest.
The spider daedra (Vaermina's Webspinner) appears to despawn because the minion spider daedra (the little one that has a shield around it) is killed. Avoid AoE damage and make sure the little daedra isn't targeted and you'll have no issue killing the big one.
I had trouble with this as well. I tried numerous times to use the scroll to reveal it but half the time it would only partly play the effect and the daedra would never appear. Then the times that it did appear it would just despawn when it's health was low before I could kill it. I'm not even sure how many times I tried this..... I finally succeeded in killing it swiftly enough that I got it before it despawned. It was reeeeally annoying. The time it worked I did focus my attack on the big one and ignored the little one.
I sat there for about twenty minutes watching other people try then give up and delete the mission... You would think they works have fixed this year's ago.. But instead they rather do 10,000 nerfs sorry I mean "adustments".. I am just going to sit there and tell everyone who comes up to attempt to complete this mission to spam the ingame support. Who knows after a few hundred ingame reports they might attempt to fix it yet again.
We have moved this thread to the bug report section of our forums. This is a relatively new section of the forum that was created for the purpose of centralizing this type of issue. As players are still reporting this as an issue we felt it was appropriate to move the thread over here. Feel free to add any additional information you have to this thread.