The party attempted to secure Wilbur Humprehy's vote on the council by completing the Find Lord Kilburn's Shield quest. But when the council voted, Slicker Silvertongue went against his master's wishes and denied the party access to the Oracle of Enroth.
Wilbur believed that Silvertongue had become delusional after spending so much time investigating the Temple of Baa, or that the Temple had placed a mind-control spell on him. He asked the party to travel to the Superior Temple of Baa and find a cure for him.
The party found a letter from Zenofex, leader of the kreegans, to Slivertongue, asking him to prevent the party from gaining access to the oracle - even if doing this exposed him as a traitor. When Silvertongue was presented with evidence of his disloyalty, he fled from the council, and the party was granted admission to the oracle.
Slicker Sivertongue can be found in the Supreme Temple of Baa in the Hermit's Isle region. The party will reach it before getting the fourth Memory Crystal. Slicker Silvertongue has the same stats and appearance as a Priest of Baa, but uses a melee attack instead of a ranged one. Spells such as Dragon breath are a good way to take him out in one shot.