Heirs Apparent is the title given to the most powerful of the Navigators in each of the Great Families. This name signifies that they may one day contend for the position of Paternova, the ruler of the Navis Nobilite, the Imperial Navigator organization.
The Paternova may come from any of the Great Families, and from any social level within them. The Heirs Apparent are usually the oldest Navigators, and those with the best developed powers, although not all develop in this way.
The Heirs Apparent are often bitter rivals who will even go as far as to try and eliminate each other if they get the chance. This sometimes leads to protracted personal vendettas or even family feuds between two Navigator Houses. The Adeptus Terra is fairly tolerant of minor skirmishing of this kind, although open hostilities between Houses are discouraged as much as possible.
Unfortunately, this keen rivalry sometimes draws Heirs Apparent into marginally unlawful or even outright illegal practices. Their personal ambition makes them vulnerable to all sorts of dangerous influence, from collaboration with aliens to dealings with the daemonic. These deviants are a minority, and most Heirs Apparent conduct their rivalries without resorting to such means.
When the Paternova does die, his successor is chosen from among the waiting Heirs Apparent, in a frenzy of violence that far outshines all previous feuds. For some unknown reason, the Heirs Apparent undergo dramatic changes. They grow larger and stronger, and the mutations that characterise all Navigators become even more pronounced. They gain the ability to survive underwater, in poisonous environments and even in the hard vacuum of space. It is said that the third eye of an Heir Apparent is enough to ward off even the daemonic creatures of the warp. Their natural aggression is increased, and they are drawn into combat with each other. As each Heir Apparent is killed, those who survive change even more, until only one remains alive. It is this vastly changed and extremely powerful individual who becomes the new Paternova.