This is the fourth of 6 battles played as a collaborative campaign over 12 weeks.  The players represent forces assembled by Grand Inquisitor Theonin to incriminate and topple the ruler of Kandran, Lord Salan.  The players have IG, Dark Angels, and Witch/Daemon hunters respectively.
As the Guard, Sisters and and DA were regrouping in the seemingly infinite labyrinth of the Palace, Grand Inquisitor Theonin radioed them:

"Allies of the Emperor, Salan has set the palace complex to detonate. You must save yourselves and assist me in stopping Salan. However, there are 10,000 loyal servants of the Emperor who will not be evacuated in time. If you wish, you may send a small force to try to diffuse the bomb, but time is extremely short and these brave warriors are not likley to be seen again. Such a force should go to Sub-level ZX, Zone Theta. The bulk of your forces and what reinforcements you can muster should meet me in the the desert wastes north of the hive."

In game terms each player was able bring 500 points to diffuse the bomb with no force organization restrictions so long as all units were legal.  The game was played as a Take and Hold with the building at the top center of the pics below as the objective.  The players started from the edge closest the camera.

The armies converged onto Sub-level ZX, Zone Theta only to find the bomb in a ruined building guarded by rat-mutants.  The mutants fired ineffectually as the Emperor's forces approached, but the Sisters' rhinos blew smoke to make sure it stayed that way.
      Hordes of the mutants charged forward, their crude firepower immobilizing one rhino.  The remaining rhino rolled forward and all of the Sisters disembarked, burning mutants with their flamers.  On the other side of the objective, two units of Assault marines screamed down from the heavens, timing their landing with Guard drop troopers.  The DA snipers on the right side accounted for many of the tightly packed mutants with a frag missile.  The Assault marines took out a strange cannon before it could fire and the drop troopers poured fire into the rat-demon leading the mutants.  
          The rat-demon wasted no time charging into the guard and tearing several aside.  The dark sorcery of the mutants accounted for several sisters.  The Assault marines swept into the objective building killing the mutants in and around it, even as the rat-demon finished off the Guard and closed in on them.
              More sisters died to the sorcery of the mutants while the rat-demon slaughtered a unit of Assault marines and reclaimed the objective.  The last Guard reinforcements swept in and their shooting along with the remaining Assault marines accounted for the Demon.  All of the mutants dead, the marines diffused the bomb just in time!