When the sun rose over the Sarangrave, the Sunbane was weaker; but when the sun crossed Landsdrop the Sunbane took full effect - even on the Sarangrave. From TWL, chapter 23:
Then Linden pointed through the branches toward the sun. When Covenant peered at the faint aura, he saw it change, just as she had predicted. The full power of the Sunbane returned, restoring pestilence to the Sarangrave
There were these "pale flagellate creatures" in the dark pools located throughout the Sarangrave - this is how the
skest were formed from them:
At ever spot where sunlight touched the surface, pale creatures rose. They thrust blind heads into the light, seemed to yearn upward. A slight wind ruffled the trees, shiftin pieces of sunlight back and forth. The creatures flailed to follow the spots of light.
When any creature had kept its head in the light for several moments, it began to expand. It swelled like ripening fruit, then split open, scattering green droplets around the pond. The droplets which fell in shadow quickly turned black and faded. But the ones which fell in light became bright -
Skipping a few paragraphs -
Hergrom directed their attention northwestward. For a moment, Covenant could see nothing through the obscure jungle. Then he caught a glimpse.
Fragments of livid green. The same green he had watched aborning in the pool of tar.
They were moving. Advancing -
Linden swore urgently. "Come on." She clinched Brinn's shoulders. "Cross. We've got to stay away from those things."