At the beginning, the Deceiver and Shiver have a little exchange. A transcript is as follows.
Shiver: Ahhh, if it isn't Alric's lapdog... will you bow to anyone who claims the Throne of the Cath Bruig?
Deceiver: The path to revenge makes for strange bedfellows, would you not agree, Ravanna?
(Shiver growls and attacks the Deceiver)
And at the end, they have another conversation.
Deceiver: The years have not been kind, have they fair Ravanna?
Shiver: I've said this before, Myrdred... and now that mouth of yours is finally going to get you killed!
Decevier: Do you mean to challenge me?!? You? A lowly camp-follower in Damas' vanquished army?
Shiver: Phelot, teach this cur a lesson! ... Fool, why do you hesitate?!?
Deceiver: Well, Phelot?
Phelot: Yes, my master.
(Phelot blows up all the Myrks.)
Shiver: What?!?
(Deceiver withers Shiver and gets blown up.)
Interesting notes on this: The Deceiver's first response implies that he's allied with Alric for his own purposes (ie to get rid of Soulblighter and Shiver). It also implies that he was allied with Connacht and whatever other emperors he was allied were also for his own purposes. Keep in mind he has a history of betraying allies, so he must have done it before the Great War, probably back when he was an avatara.
The second part of his response is equally interesting. It implies that Ravanna also has a strange alliance in order to get revenge. The alliance he speaks of is obviously the one with Soulblighter. I would guess this would be odd because they used to be enemies back when Damas worked for Connacht, since she's an ancient evil and all. This would also tend to contradict the theory that Shiver and Ravanna are seperate people and Ravanna was impersonating Shiver. There never was any real evidence for that theory anyway, but I suppose I'm a fine one to talk after that stupid theory I promulagated before. If this is so, there would be nothing particularly odd in this alliance. This could boost the Damas and Culwyeh used to do it theory as well.
As for who's she's getting revenge on, it can't be the Deceiver, since she joined with Soulblighter before he turned up. Therefore, it must be Alric or the light in general.
The section from the end also contains much interesting information. We know they've had problems in the past, but it's not clear whether it was when he was an avatara or during the great war. It's also not clear whether she gets mad based on him calling her ugly, or calling her Ravanna. My money's on the ugly, since he already called her Ravanna, and she's already mad about that or possibly what he was insinuating about her relationship with SOulblighter. Ugly broke the camel's back, to use an already overused phrase.
If she wasn't really Shiver, I see no reason for her to attack him at this point. She should have the sense to know she can't win. However, vanity is Shiver's weakness, so she could fall for such a thing.
He also taunts her that while she was the most powerful sorceress around at one point, she's been reduced to being a lackey for a guy who's about to lose. So basicalyy, he hits her with a trple-whammy. She's not beautiful anymore; she not powerful anymore and he knows her secret. This is what goads into doing a dumb-ass thing like attacking him.
Phelot was obviously bound before a Murder of Crows. Since Shiver didn't have him then, it explains why she didn't suspect anything. It also shows that the Deceiver can convert Shades, though it takes extra effort, and the converted unit can hide it very well afterward.
Also, once he kills Shiver, she's out of the way, and Soulblighter's defeat is imminent. His revenge is complete, and it's time for him to make his exit. Boom!
I think he does have some loyalty to Alric/the Cath Bruig though, as his actions before the Wall don't make any sense otherwise. I don't think it was a matter of wanting to kill Shiver himself, since he has no problem letting Alric kill Soulblighter.