Velk'Han is a Sept of the Tau Empire in close proximity to the Jericho Reach. It is also one of the furthest Septs away from T'au and is considered by many to be a great display of the prowess and tenacity of the Air Caste to not only reach, but also settle. Unfortunately, this Sept is also a great source of anger from the Imperium of Man and is a key reason as to why the High Lords of Terra commissioned the start of the Achilus Crusade.