A Fenrir is a massive plague construction. So large, that it can eclipse the sun for a helpless planet. It is a beast that devours all life it can find within a system, adding to its mass. Ships. Soldiers. Marines. Anything and everything is absorbed and integrated. The largest Fenrir recorded was approximately [~6 × 10^25 kg] in mass, the size of a planet. It destroyed noble fleets and ripped worlds apart. It took many, many fleets working together to destroy this monstrosity. Thousands of ships were ruined. Millions of Empyreal Navy-men and Privateers died. Even the Nobility leading the attack were so grievously wounded that they did not survive the return home. The star that the battle took place in had to be detonated with Noble Technology, so great was the Fenrir.
Fortunately, most Fenrirs do not possess much in the way of intelligence or long term planning. However, this one was special. Based upon historical and prescience data, it was determined that this Fenrir grew around an Ascended Plague Lord. Plague Lords spontaneously arise when the Plague evolves. It eventually takes on a human-esque form, gaining willpower and further intelligence. Usually, these can be defeated with overwhelming firepower, just as any Ascended can be defeated.
However, should an Ascended, live or dead, fall prey to the Plague, and be taken back to the Place Of Origin, the Plague will undergo a rapid mutation. A Plague Lord, usually something taking many, many thousands of years to form, will nearly spontaneously appear. The Data stored within an Ascended will be used to modify the Plague. Willpower is based on the amount of certain nano-cells within the body -- however, the Plague will create these cells at a rapid rate. It will no longer bound by the restrictions placed upon the Plague by the Endless Emperor/Empress. It will be an unfettered demon, capable of devouring worlds -- reshaping them with Willpower. It is recommended by all involved that during battle with the Plague, that all Ascended corpses should be treated as relics or destroyed.