It’s thought that the Tyranids were once a species like any other, whose knowledge of biological engineering advanced until it consumed them, turning them into the ravening horde of galactic locusts they are today.
We can see echoes of the original Tyranid form in the six-limbed and reptilian structure of many of their bio-constructs. They may be insects now, but once they were dragons.
There must, then, have been a time when the primeval Tyranids had not yet formed the hive mind, when they made the decision that led them to alter themselves in such a terrible way. Was it an involuntary fall, like that of the Eldar? Or a desperate gambit to avoid some greater threat, such as Chaos? Did they flee something even more terrible in their home galaxy?
There are none now who remember. Unless – one of these primeval Tyranids saw lucidly what was to come, and rejected it? Unless he fled, always keeping one step ahead of the Shadow in the Warp, out of the grip of the hive-mind? Tens of thousands of years in hypersleep, millions of years of relativistic time hurtling away from his devoured galaxy, the hive fleet snapping at his heels.
Old, this space dragon, old beyond reckoning, skin like paper, translucent and bleached by time. Fierce and stubborn, the last to still refuse the call. He’s the last of his kind; the only vessel remaining for the culture and knowledge of the old Tyranids. He refuses to die, just as he refuses to become an insect crawling in its hive.
To human eyes he’s a dragon, ancient, jealous and fierce; and if not a dragon, an old alley-cat, tired but ornery and vicious and possessed of an implacable will to survive. He has drawn to him a warband of other misfits and genetic cast-offs; a Genestealer Patriarch unwilling to see his brood fulfill their destiny as food for the hive fleet; a surviving Zoat, considered obsolete by the hive fleets; a mercenary Fomorian.
This, then, is the Last Tyranid.