Once you get Dythra down below half health (Need to confirm exactly) s/he calls in ALL the mobs in the zone, range is not an issue, so you need to clear the zone before taking Dythra.

If the raid wipes in the instance you will be forced to revive outside the zone and be locked out. Prevent this by camping a healer in the zone to bring in incase of a full wipe or running to the entrance and staying along the zone point will break aggro.