There are times when there's a lot happening in veteran Asylum Sanctorium if you're doing +1 Felms or +2 Hard Mode that you simply just don't see the Felms AoEs come rushing out of their explosions.
If you want to turn this:
All you have to do is make sure you have ESO turned off then in your files (if Windows), go to:
Documents // Elder Scrolls Online // Live // UserSettings.txt and then look for:
SET DETAIL_MAPS "1"
and turn that 1 into a 0 (zero)! So it should look like: SET DETAIL_MAPS "0"
Make sure to save the document then relaunch the game!
This change has absolutely no impact on your game's performance whatsoever and shouldn't cause any adverse issues. The only other visual effects we've noticed that are also amplified besides Felms' wave AoEs are the auras around the S'kinrai and Vashai adds in Maw of Lorkhaj.