Clearly there are a multitude of ways to work this in, from excerpts of the Books of Harrow to a dragon seeking a new weapon, perhaps having a second group tailing the first and collecting True Elements while the players collect Horror Elements. The dragon can then later bring the players in on the secret because the second group needs some saving.
Personally, in a campaign that ends with curing a Passion, I'd make Passions and their Questors center.
Questors of Garlen, Astendar, Mynbruje, etc., begin employing the players. Eventually, a Questor of Lochost puts them on the trail of creating Sufferance and Renewal, as a way to free the Passions enslaved by madness. At this point the players are probably used to Questors giving them jobs, but I'd play up the oddity of a LONE Questor seeking them out. This person also stresses the secrecy needed, as they fear one of the Mad Passions or Horrors finding out.
This Lochost Questor is actually a Questor of Vestrial who thinks Vestrial wants Lochost driven mad. In actuality Vestrial seeks to cure Raggok. Because reasons. (Perhaps a long-term plan to overthrow Horrors and take their place, or, ya know, Vestrial's nuts.)
Through their work, the Vestrial Questor is revealed. Ideally, this brings the players into contact with a Questor of Rashomon, or a player becomes one.
Yes, that's a Questor of RASHOMON.
As things move forward the players get to meet at a council of the Passions, something never before done. All of the sane Passions are there, plus one Questor each, and the Questor of Rashomon. They plead with the players "to cure our lost brother Rashomon", even though Vestrial wants it. Dis crashes the party and exclaims great rewards to whomever kills Lochost, or drives her Mad.
Later a Questor of Raggok, a skeletal figure wrapped in death and maggots, attempts to persuade the players to go after the other Passions, even agreeing to reward them if they help Dis.
Vestrial contacts them again near the close, crazily requesting, again, that they cure Raggok. His only reasoning he'll give is that he's always wanted to kill a Great Dragon.
If the players take on Raggok, the follow on adventure has Rashomon Questors leading efforts against Iopos.
Anything could happen, even curing Vestrial or just killing Astendar.
This all works even better if players are Questors, and it becomes even more likely that they will become ones during this adventure. If one becomes a Questor of Rashomon, all the better.
I'd even throw in Questors of Erendis and a sane Vestrial. They are possible because of the POTENTIAL of them being healed. When the players complete the quest and Sufferance and Renewal are destroyed, then the Questors of still Mad Passions are themselves driven mad and serve the Mad versions.