Found a guide that mentions this. The guide refers to similar mobs in Unrest, but says they're also found in Najena at lower levels.
"They aren't all that hard to kill, but you'll want a group, because TTs are casters, either shadow knight or necromancer class (though they didn't have any pets, and they didn't harm touch that I could tell). They cast lifetap- and disease-based spells."
"Notes: They can be dangerous in large numbers; they cast DoTs that will suck your life away. If you are 45+ you should have no trouble, but be careful anyway, for you can still get overwhelmed"
Source:
http://www.magecompendium.com/orbofmastery.html