It seems to me that Sili may be right, but here's my little contribution.
Soulblighter figured out that Shiver was indeed Ravanna sometime after his "summoning" of her. This was probably near the dam episode. He tries to kill her by smashing the dam and flooding the valley, killing two of his old enemies with one fell swoop. Consider these quotes from the texts:
"The scouts told us that Alric and nearly three thousand men from the Legion have come from Muirthemne to face Soulblighter. Unfortunately, they were met by Shiver and her army in the valley about two hours downstream from the dam. If the dam were destroyed, the resulting deluge would kill everything in its path for miles."
-The Wall Narration
"Still Shiver stands between us and Soulblighter, just as she did two days ago on the Ire River. The men who fought there faced an army of thrall meant more as an impediment than anything else. Did Soulblighter plan to wash both the Legion and Shiver out to sea? Truly there is no end to the fiend’s malfeasance."
-Shiver Narration
So consider this, Soulblighter tries to slay both Ravanna and Alric at the same time, but fails. Ravanna doesn't know about his duplicity because it was soulblighter's troops that tried it and the light isn't about to tell her. So she is pushed farther and farther back towards her master.
When the Deciever meets up with her in the barren lands near Silvermines, she's at her wit's end. She's sandwitched between Soulblighter and Alric with no way out. She normall probably would have set up a bunch of traps and made tracks back to Soulblighter's camp near Tharsis (and probably would have died at his hands). Instead, during her little conversation with the Deciever (presumably just to gloat a wee bit before hightailing it away) he brought up her alternate identity, just as Sili said. The rest is history.
The main point of this was to show that Soulblighter actually tried to kill Shiver without her knowledge, eliminating you commanders is a stupid move unless you've got a vendetta against them...
-Disc
