Two briarvexes who work in conjuncture can cast the spell Entangle that lasts for 1 round. Normally two Briarvexes work together and recast that spell all the time so even if someone breaks free they are entangled again. Briarvexes are immune against the Entangle effect of other briarvexes.
If they hit someone they can deal additional damage by spending a swift action that simulates the briarvex planting a thorn in the wound from their first hit and let the thorn grow as a swift action.
It seems Briarvex are either minions or are significantly more powerful.