What you have already looks pretty good to me, I don't really know what to say- if I knew anything about writing, I'd have done it myself. No offense to you, but I usually make my stories a lot more detailed- which is why they always turn into epics, I guess. If you want something to go on, here's the story outline I thought up and posted back on page 37.
Intro; explains how the planet was colonised by Tau, invaded by Imp Guard, and that the Tau fought them off. Then bizarre murders/disappearances started happening, and before the Tau realised it they were under siege again; this time by what they discovered was an army of the walking dead.
Tau intro; combat seen through the eyes of a Fire Warrior as he fights in a ruined city besiged by the Undead, he recollects on his role during the early parts of the conflict; he and his squad were sent to a reaseach center which high command had lost contact with. In the forest outside, they were attacked by a Carrion Swarm, in the form of a pack of flayed dogs, who killed one FW and chased the survivors into the center, where they came under seige by the Undead (including a few Tau zombies). All of the others died, except the Tau protagonist, who escaped after setting a self-destruct mechanism. He'd thought he'd curtailed the Undead threat. He was wrong (obviously).
Necro intro; a look through the campaign via the eyes of Khalgar; we find out why the Necro's came there, get his take on the personalities of the other Necromancers and find out that this one city is all that remains of the Tau forces; the Necromancers have conquered everywhere else on the planet.
Story body; Vau'N discovers Khalgar, Lucius and Samael planning their next assault, having just wiped out a squad of FWs and a group of Kroot. Vau'N snipes at Khalgar, getting a single clean shot with his weapon that punches a hole straight through Khalgar's chest. His elation is cut short when Khalgar simply staggers back a few steps, then turns and looks right at him, hurling a Death Bolt that smashes the spot where he was just a few seconds ago. Vau'N flees, Lucius and Samael start after him but Khalgar orders them elsewhere; Lucius is to find and smash any remaining communications tech and Samael is to hunt down and kill the Ethereal, thus shattering the will of any Tau survivors. Still fleeing blindly, Vau'N stumbles across a batch of Tau Ghouls feeding on the pilot of a battlesuit. One running battle later, he heads for the space port, simply hoping to meet up with reinforcements there. We cut to Sameal, as he sneaks into the Ethereal's compound and massacres the occupants (graphic or implicit violence? it's your choice). A few more 'jumps' in PoV as we witness a group of Fire Warriors being chased away from their post by Ghosts, the last remaining pack of Kroot being overwhelmed and eaten by zombies and an Undead Beast (Carnifex) smash its way through a cluster of Hammerheads. We then cut back to Vau'N at the space port; wounded and exhausted both mentally and physically he is attacked by Lisbeth, who knocks him unconcious. With his last ounce of strength, he hurls an iron spike at her, piercing her heart just as she prepares to use her psychic powers; shocked by the impact she loses control and is consumed by warp energy; engulfed in witch-fire she burns away to ashes and is scattered on the wind.
End; at the space port, Lucius asks Khalgar if he's going to join the others in 'carving up the spoils', in dividing the dead and the survivors of Lisbeth's forces (she is permanently dead now) into their own forces. Khalgar says later, and waves Lucius off. After Lucius leaves the scene he examines the dying body of Vau'N, and decides to turn him into a Harrowed as a mark of 'respect' for his prowess in battle and his skill in surviving. Vau'N wakes up extremely confused, as Khalgar welcomes him into Undeath.
That's all I can offer you, except for some insight into the natures of the Necros; Khalgar is old and jaded, Lucius is like a stereotypical teenager- hating everything and everyone, including himself and Lisbeth combines the stereotypes of the Goth and the newly-turned vampire- drunk on her power and in love with what she's become. None of the other Necromancers like her, so they don't shed any tears when Vau'N kills her. Samael is almost as old as Khalgar, and like most Phagi has completely lost his mind; he's little better than a blood-crazed carnivourous animal, running entirely on instinct.