Okay, spoilers below, so don't read this if you don't want to know what's going to happen...
In Lords of Oblivion, the players rescue a spy who works for the Striders from a Last Laugh hideout. In so doing, the players must face Jil as well as a Jester, on the the leaders of the Last Laugh.
The spy reveals that Thirifane (Vhalantru's wizard helper from Life's Bazaar) is having a big meeting of various baddies. During this meeting, Thirifane reveals the Cagewrights plans and explains a little about the Ritual of Planar Junction.
The players disrupt the meeting, and follow the lead to Vhalantru's lair (Oblivion). Vhalantru meanwhile has joined himself with a Demodand spirit, and has become even more powerful. The adventure ends with Vhalantru's destruction.
So this is obviously a big event in the Adventure Path series. My only question is: what were the Secrets of the Soul Pillars again? I never did
figure that out from the last adventure...