The tide of battle shifted every so suddenly. Ser Fieldcarer's men were able to take steps forward, unmolested, to choose men to cross swords with. But those chosen men were distracted in their entirety. They were looking behind them, rather than to the front. It was an easy enough task to slip a spear under their awareness, and steal an easy kill on them.
It was not until half the men against them had been culled that Tiller found out the reason why. They seemed to be slain by some sort of invisible hand, as if it was disease killing them, rather than men. And when he did finally catch sight of the men doing the killing, from the twisted look on his face, and the stooped nature of his old body, disease certainly seemed to be an apt way of describing him.