You need detect invisibility or detect greater invisibility to see these
First you need to travel to this location in duskwood
Once there you will see the "unseen"
You can attack and kill these mobs, they drop a few silver and sometimes can drop greens.
Also, if you go there even without detect invisability and use an AoE spell you will be attacked by them
Have fun!
Dunno to put it here or in exploration, its basicly not rly giving you any advantage, but its pretty cool ^^