Once the amplification crystals are in place we can begin the process. The amplification crystals are only needed for the possession of Greenhill. Once the dro-m'Athra have gained their foothold here, the artifact will allow them to spread their influence throughout all of Reaper's March.
Still, we should try to keep the crystals active for as long as we can. The more villagers the dro-m'Athra can possess, the quicker they will be able to spread their influence.