Derkhan was staring into the darkness with a look of astonished hope.

“Gods bless you,” she screamed down the slate, into the night. She fired again with her left hand, but the bullet passed loudly and harmlessly into brick.

Thirty feet below them, the injured man still screamed. He fumbled ineffectually to undo his mask.

The unit split. One man ducked beneath outcroppings of brick and raised his rifle, aiming into the darkness where the newcomer hid. Several of the remaining men began to descend towards their new attacker. The others began to climb again, at redoubled speed.

As the two little groups moved up and down across the slippery roofscape, the dark figure stepped out again and fired with extraordinary rapidity. He’s got some kind of repeating pistol, thought Isaac with astonishment, and then started as two more officers reared up from the roof a little way below him and fell, twisting and screaming, to bounce brutally down the incline.

Isaac realized that the man below them was not firing at the militia who had turned and were approaching him, but was concentrating on protecting the little platform, picking off the closest officers with superb marksmanship. He had left himself vulnerable to a massed attack.

All across the roof the militia froze at the volley of bullets. But as Isaac looked down he saw that the second group of officers had descended to the base of the roof and were running in clumsily furtive formation at the shady assassin.

Ten feet below Isaac, the militia were closing in. He fired again, knocking the wind from one man, but failing to penetrate his armour. Derkhan shot, and below them, the poised marksman screeched an oath and dropped his rifle, which slid noisily away.

Isaac filled his gun with desperate haste. He glanced over at his machinery, saw that Andrej was curled under the wall. He was shuddering, with spittle fouling his face. Isaac’s head throbbed in time to some weird beat from the growing blaze of mental waves. He looked up at the sky. Come on, he thought, come on, come on. He looked down again as he reloaded, trying to find the mysterious newcomer.

He almost cried out in fear for their half-hidden protector, as four burly and heavily armed militia jogged towards the pitch-shade where he had hidden.

Something emerged from the darkness at speed, leaping from shadow to shadow, drawing the militia’s fire with extraordinary ease. A pathetic spatter of shots sounded, and the four men’s rifles were empty. As they dropped to one knee and began to reload, the cloaked figure emerged from the sheltering gloom and stood a few paces before them.