Karn racked his sniper rifle, and fired. The shot hit, but deflected harmlessly off power armor. The traitor marine howled, his voice distorted by the grotesque helmet. Karn ducked, as his position was showered by bolt shells. He scurried over to another balcony, and looked around. From the neighboring hab-block, another sniper scout held the traitors back for a moment with supressing fire. Then he too had to duck, as the tank opened up again. Although Karn never considered Rhinos to be anywhere near a main battletank, this foul machine, decorated with blasphemous sigils and symbols that made the eyes hurt, was outfitted with some sort of frag launcher and one of those powerful bolters that the marines also carried.
Using the momentary respite, as the enemy concentrated on the other hab-block, Karn stuck his head out. He identified the positions took up by Alpha Squad in an instant. Their situation was dire. The vox was useless, spouting in turn blasphemous curses or ululating cries. Multiple scouts had been incapacitated, or otherwise severely wounded. The enemy was pressing them hard. He saw no way out. Except...
It must have been the Emperor interceding on their part. Karn saw no other explanation. After the final fusilade, the enemy weapons fell silent almost as one. The chaos marines reloaded. The near silence was eerie after the continuous firefight, and it wouldn't last long. Karn inhaled, then shouted at the top of his lungs.
'Sigma-9!'
He ducked back immediately, and his position was again battered by bolter fire. The surviving cultists added their meager autoguns to the onslaught, and a moment later, the frag launcher on the Chaos Rhino thumped and fired as well. Karn fell back. He could only hope that his squad mates had heard him, and had understood his meaning. He would provide them with some cover - wild shots fired while running... as much as he could muster while making his escape.
Two days later - Emperor knows how - Alpha Squad had reunited at grid point Sigma-9.