HIVE FLEET IMP
Ever since their flesh was taken, select necron dynasties have scrabbled to go back. Back to when they could feel, truly feel.
Some dynasties also wished to maintain their absolute control over their subjects, so they could continue to revel in their new blood and bone.
His majesty, Lord Neketekh the III wished for both.
And so ships set out across the grand void, tiny pips of regal discovery against the cruel galaxy, against many foes and against all odds and hope.
Lord Neketekh had awakened earlier than many, and wasted no time carving a small feifdom from what this Imperium called theirs. Naive fools, crying to gods.
His explorations found no suitable host for His dynasty and to make matters worse, the Imperium was pressing back harder and harder with each passing year, the need for revenge fueling them as much as faith and the need for materials. But his warriors could hold.
When the Hive Fleet Behemoth stormed the Imperium Neketekh saw a small dot of hope for the reversal of biotranseference.
He decided that the Great Devourer would make a grand host for His lordship and His vassals.
He captured specimens and began His experiments and over time, in conjunction with samples from other fleets had made a perfect host. However, the attempts to put the minds of his kin into these bodies were all unsuccessful and were taxing, requiring energy and the permanent death of some of his lower castes.
The Imperium had seemed to accept His dynasties presence, the systems He held not being worth the effort or focus to take back, the Imperium staffed a sizeable garrison to keep His expansion in check.
In turn Neketekh set his own garrisons and armies and the two powers waited.
Things had been going auspiciously so far and would have kept going that way had a genestealer cult not infested some of the nearby Imperial worlds and awakened some dormant drive in the captive tyranids to begin spreading. Neketekh had installed only a small token force at his reversal laboratories on his core worlds and had to draw troops away from the borders to contain the new threat.
This then provoked the Imperium to attack, foolishly portioning the genestealer cult among their ranks. Defeat and a legacy of ashes in sight, Neketekh the III wailed for help, but none came for Him.
When agents of the Administratum came to collect tithe, they found only the legacy of civil war, the shadow in the warp and cult sabotage having prevented call for aid and the dead ruins of a long dead race fashioned in the shape of the dead.
Hive Fleet Imp fights using tactics learned in their initial conflicts. Getting hit with gauss weapons destroys biomass so The Imp uses stealth to not be seen and to safely access biomass.
The Imp also seems the have been gifed the ability to hijack other fleets cults and uses them to its own will.
Imperial adepts educated guesses as of M42 pins this Fleet as active in the north of the galaxy, looking to build up biomass.
TL;DR, necron dynasty tries to use tyranids to return to the flesh, gets jumped by escaped tyrands and the Imperium, tyranids learn to use stealth and use other fleets genestealer cults.