There's a lot of excellent advice on these boards about what to do with the main villain of this AP, especially how to introduce him earlier. I've taken on board a lot of it and I just wanted to share a little about how I've gone about integrating him into the story in my own ongoing PbP game here on the paizo site.
Well the first mention for me was in book 1 where Kendra got a letter of condolence from Adivion and apologies that he couldn't be there in person. He also said he was working on a position for her at the University of Lepistadt having some influence with the Dean there. This provided a hook for book 2 where he actually showed up in person. He, along with Judge Darramid, was the main driving force for the party taking on the Trial of the Beast. I utilised the FANTASTIC LETTERS FROM THE MAIN VILLAIN idea that Loki_Thief came up with and the first of these arrived at the end of Book 1.
Adivion offered support and advice and befriended the party throughout the Trial and was even present during the riot, helping to calm the crowd. Later, Auren Vrood was still present in the Schloss when the party arrived and was actually working on the machine whilst inside a force cage. The Beast turned up and a huge battle ensued with the Aberrant promethean afterwhich Vrood finished his work on the machine and took control of the Beast who attacked the party - leading to a very dramatic and heartbreaking choice of whether to destroy the creature they had befriended throughout the Trial. Following this the party returned to Kendra's house to find the place in disarray. Kendra was gone and Adivion lay dead.
What I really wanted to do was to constantly drop hints that Adivion was behind it all and the best way to remove himself from suspicion was to have himself killed (only to be later resurrected which also shows his level of planning, intelligence and resources). There were clues from the vision at the Stairs of the Moon, Lady Ivanja in the Lodge, the letters Adivion was sending and dialogue between characters.
Throughout it all I've wanted to portray him as the man behind the curtain, pulling all the strings. This culminated in the confrontation with Vrood in book 3 where the spirit retrieves his memories and showed adivion ordering Vrood to kill him and Kendra in order to put the party on Vrood's path. This was to punish Vrood for killing his friend Prof Lorrimor and also to distract them from his real work which was setting up the exchange of the Raven's Head in book 4.
This all worked beautifully. The party were convinced for 18 months in real time that Vrood was behind it all and were surprised when they managed to confront him and defeat him in Book 3 but when the spirit revealed Vrood's memories and the fact Adivion was behind it all - the player's were stunned. I then posted a huge reveal that tied together all the clues they had received in a kind of MOVIE FLASHBACK and the players were taken aback at how Adivion had weaved the whole thing together. He is rapidly becoming the best and most engaging big bad they have ever faced in their years of gaming and it's thanks mostly due to the fantastic writing in this AP and the advice I received on these boards.