They are definitely harmful to the scavenging operations, but I believe they are great to have around.
For starters, they chased the bees away.
And they are not aggressive as long as you don't get in the way of their work.
But what is most fascinating is their work; they weave space time into threads that stabilize reality. That disables warp technology but also appears to have a countering effect to extradimensonal interference. The down side is that they often destroy or disable rooms that are connected to their webs.
Could they be from a dimension opposing to the demons? Or could they have been engineered by the old civilizations to combat their cunning invaders?
Effect
If you block all disaster damage from this, you have to sacrifice 2 rooms.
Damage
If (X) is the number of disasters played before this one, then this causes:
A- 1+X 
damage.
B- 1 
damage.
C- X 
damage.