Great thread. I have always assumed:
1) Lord Foul wasn't called "Lord Foul" when he was on the Council. The betrayed Kevin or others named him Foul after the revelation that he was an enemy. LF kept the name because it gave him a chuckle.
2) He called himself Lord a-Jeroth or Lord Jeroth when he was on the Council. I believe SRD confirmed it in the GI.
3) He had a bloodguard. LF probably killed that Haruchai himself when he revealed his true nature.
4) He rode a Ranyhyn. Foul surpasses the Earthpower and can pervert or circumvent it. Same with the orcrest and high wood. His true nature was hidden even from the great horses.
5) Kevin had suspicions, but not because of something Foul did. In fact the text states he did nothing evil "that Kevin could discover". The suspicions were not based on LF's deeds, or Kevin's lore, or Earthpower. The suspicions were pure old intuition and gut feelings. Hence Kevin's distrust of them and shame at feeling them.
What *I* would like to read is an account of what happened *after* LF revealed his evil but *before* the Desecration.
Imagine that conversation between LF and Kevin, when K challenges him to the Ritual, and LF agrees.
LF knows it won't destroy him, but doesn't want Kevin to know that. Kevin would never do the Ritual if he thought Foul would survive. Even if he knew Foul would be diminished for 1000 or 2000 years, he still would not (I believe). Kevin had to think the Despiser would be undone by the Ritual.
So imagine Foul's brilliance in agreeing, but not seeming too eager. He can't tip off Kevin that he (LF) will survive. He has to trick Kevin into thinking that he (LF) hates the Land, the Lords, and Kevin himself so intensely that he is willing to kill himself to wreck it and destroy the Lords.
Shades of "we are foemen, you and I"...LF's lies are so brilliant, because there is a shred of truth in them. LF *does* hate the Land, the Lords, the Earth...so very much. And the Ritual *will* hurt Foul. It will reduce his power by maybe 95%. It will cost him dozens of centuries of time, time he will be imprisoned under the Arch, without even the balm of his power to soothe his anger and frustration.
So when he talks to Kevin, this kernel of truth disguises his deceit. He knows the Ritual can't undo him. If there was the slightest chance of his own death, Foul would never enact it, because he values himself far more than he wants to destroy Kevin or the Land.
I'd love to see how he tricked poor Kevin into doing the Ritual. And how long did that Ritual take, anyway? Minutes? Hours? Maybe days?? Picture the two of them, under the mountain, locked into the Ritual...interacting? Chanting together? Kevin doing a part of the Lore, then turning it over to Foul, then Foul stepping back as Kevin completes another part of the no-doubt intricate and complex magic?
Remember this is not something Foul could do himself.
And think of the journey Kevin made to Kiril Threndor. He went alone, right? Foul was already there. What did Kevin think about, as he rode to his death? He knew he was going to die. The animals he passed were going to die. His horse under him was going to die. Pretty much every living thing in the Land was going to die..and he was the one who was going to be responsible!
I'd love to read a good fan-fic treatment of this subject!
p.s.
Were the Viles still under Mount Thunder when the Ritual was being enacted? Demondim? Surely ur-Viles. Wonder if they knew what was going on above them (Kiril Threndor is far above the Lost Deep, right)?
What about SHE? Did she have any knowledge of what those two (LF and Kevin) were doing up there? Would she care?