This isn't actually a Sunwell daily quest -- it's a one-time thing -- but it is done on the Isle of Quel'Danas, and it does open up more dailies (keep an eye out for posts on "Blood for Blood" and "Blast the Gateway"). "The Missing Magistrix" is picked up from the Shattered Sun Staging Area, and asks you to go look after an operative gone AWOL by means of a portal and a scroll. You do need a flying mount to get this quest, since if you didn't have one you won't be able to get back from where you're sent without burning your Hearthstone.
Finding this portal seems to be very difficult for people, so I'll provide as much information as possible.
- The coordinates are 48,44
- It's south and very slightly east from the Staging Area
- It's subtly indicated on the map in the screenshot
- It is a wall of green glow similar to the Dark Portal into the Outlands