The ethereal golem is a spiritual mockery of a stone golem in Dragon Age II
Involvement
It is encountered during Varric's companion quest Haunted. It appears to be some variety of Spirit or Demon which has taken on Golem form, as it is capable of splitting itself into multiple Shades before reforming.
It's appearance in Bartrand's Estate seems to be linked to the shard of Lyrium idol left behind by Bartrand.