Kevin's Watch

The way I understand it, Caer Caveral puts the location of the One Tree in Covenant's mind.
SPOILER ALERT:
Covenant wants to find the One Tree in order to make a new Staff of Law, but Vain is actually destined to become the Staff of Law.
Thus "the thing you seek is not what it appears to be."

If I remember correctly Vains purpose was pretty telecast at the end of the One Tree in that his arm was turned to wood by contact with the 'eldrich power' emanating from the One Tree - that and the fact that he was wearing the heels (not high heels I hasten to add) of the original staff. The only suprise as I remember was the incorporation of Findail. But presumably the visit to the One Tree was absolutely neccessary to begin the transformation process. Did Covenant and Linden not realise Vain's purpose - I don't believe they did. They must have been the only ones who didn't though.