It's supposed to be based on the caster, but there's a bug with the set now where it seems to choose a random person in the group to center around. My team tried to use it in vAS and the procs were really strange, sometimes hitting the back kiter by herself who was well out of both the boss's and the set wearer's 28m range, and sometimes hitting only a few people in the group, even though a DD in the group was wearing it.
Due to the age of this thread, and the fact the information within is 2 years old now, we have gone ahead and closed it down. Please feel free to create a new thread for discussion on this topic if you wish to do so.