One of many different types of portals, the Circle of Thorns uses this portal to summon Behemoths, Behemoth Lords and Behemoth Overlords. Many of these portal types that summon Circle of Thorns enemies can be destroyed to prevent further summoning.
Other times Circle of Thorns portals will be a means for you and your team to travel to a differant section of the map.
Certain, bugs do exist with these portals. One bug that seems the most common is passing through with no effect. in other words it does not take you anywere. In most cases this is caused by players that have super speed turned on.
The Circle of Thorns have opened this portal to another realm. Who knows what will come through. It must be destroyed!