First set of mobs in the ring event is spiders, they are not grouped. Lightweight.
Next set of mobs int he ring event is skeletons in groups of 3. They hit a good bit harder.
Finally, when about 2-3 skeletons are left, De'manti Sheda will spawn.
De'manti has a fear, and also has a buff on him that will heal him 10% when he does an offensive spell attack (though it's not clear if every attack did this).
Tried this twice last night, both times it reset as we were killing the skeletons. We've downed him before, so not sure why it reset. It was near the end of the night so we just called it. Anyone else have the encounter reset randomly?
Only way we've found to reach this mob so far is to evac. In order to start the ring event someone has to click on the orb above their head. Straight burn with no real issues. As mentioned before the skelly's do hit hard but they are managable. Also you can evac at any time and kill this encounter 
Straight burn, as previous posters has suggested, have a tank pick up the adds as they come and burn as many as possible before the named spawns. Flip the named and do a straight burn.
Did not get hit by the fear (due to fearless morale crusader AA), and had no problems tanking all the adds + the named as an SK at 130 crit mit.
We intentionally wiped to him the first time we found him - 7 out of 12 of our raid force had called out to guild hall, and our group evac'd. Had him down to 50%, but he healed back to full.
After we wiped to him, we found we could re-trigger the ring event, but the named didn't seem to re-trigger.
Once you have killed him successfully, the fear orb can be clicked to get you back out of the fear cage.
Nice Addition guys.