Miss Mayhem by Rachel Hawkins

 Several months have passed since Cotillion and Harper is working with Ryan and David to help figure our David's powers without getting him killed or making him crazy. The friends have decided to try dampening David's visions (without his knowledge) because the visions are so painful for him. This seems to work until one night when David has an extremely powerful vision that leads him to the headquarters of the Ephors (the people tasked with overseeing the Oracle and who up until recently wanted to kill David), a home which up until weeks ago had been a burned out wreck.

At the house they meet Alexander, one of the Ephors who reveals to Harper that she is not technically a paladin yet. Alexander informs Harper she must go through a series of trials called the Peirasmos, and if she doesn't do them, then the Ephors will simply keep coming after her and Ryan until they are both dead and they can make a new mage and paladin. Also, she doesn't actually have a choice in whether to DO the Peirasmos, because the trials officially began as soon as Harper set foot inside Ephors HQ. They also produce Harper's best friend Bee, who has been with the Ephors since Cotillion, and tell Harper that if she dies, Bee will take her place is paladin. And, so that Harper gets no extra help, David has been temporarily stripped of his Oracle powers. 

Back at school it turns out Ryan's spell to make everyone forget Cotillion was much stronger than they realized, and half the school doesn't remember Bee at all, including her boyfriend Brandon. In order to find some kind of normalcy in this new life, Bee suggest that she and Harper enter the Miss Pine Grove beauty pageant, something Harper things is super tacky. However, she decides to it in case the Ephors plan to do something dramatic at the event, like they did at Cotillion.

On a day of a school assembly, fire alarms suddenly go off and all of Hareper's spidey-senses start tingling. Someone used gasoline to set the English department on fire, and David is trapped in the newspaper office. Harper is able to save him, but not without knocking out her principal. Ryan tries to work his forgetting mojo on him, but for some reason it does not work. They soon learn this was the first Peirasmos trial and Ryan's abilities have been deliberately "turned off" so he can't help Harper in any way. Alexander also tells David that Harper and Ryan had been teaming up to stifle his visions. 

Harper and David have a massive fight about messing with his powers and they break up. Harper also narrowly avoids being expelled for beating up her principal. A few days later, at the local fair, the second trial arrives. In the funhouse Harper sees horrific images of Bee, her parents, and other people she loves being injured or dying. The final vision of Harper herself, murdering a version of David who has lost control of his powers. Afterwards, Harper learns the test was set up to make her see all the things she fears most and was meant to test the strength of her mind in the face of all her worst nightmares. 

After several days of not speaking David comes to Harper and tells her that despite Alexander's wards, he has been able to have small, blurry visions, and thinks that if she and Ryan help him, he could potentially break all the wards, and maybe even break free of being the Oracle. However, when they try to force a vision David undergoes a tremendous and terrifying surge of power that leaves him completely shaken. He refuses to talk about what if anything he saw. Harper comforts him and they share a night together (not graphic, just implied they have sex). The next day Alexander demands to know how David accomplished the vision and what he saw. When David refuses to tell him, Alexander cancels the Peirasmos.

As David and Harper leave the Ephor house, they realize it looks much shabbier than it had before. David also tells Harper that as much as he wants to, they should not get back together because he has seen a vision of his powers corrupting him. 

On the night of the beauty pageant, Harper notices Bee is acting a bit off, but she writes it off as nerves until Bee attacks her just before the talent competition. Harper assumes Bee is working for the Ephors but Bee reveals she is actually helping David, who asked her to distract Harper so he could leave town. Harper tries to go after him but Ryan, who has also agreed to help David has put rune locks on all the doors to the rec center. Unfortunately, in order to leave, David had to destroy all the runes keeping him in the city, which causes massive destruction all over town. The Ephor house has also reverted back to the abandoned wreck it was before Alexander took it over. 

Desperate for help Harper turns to Alexander, to beg him to get the other Ephors to bring David back, but Alexander reveals he is the last of the Ephors. The magic of the Oracle is what sustains the Ephors life force, and with David being hidden from them for so long, all the other Ephors died except for Alexander. He is also now dying, having only survived by killing Blythe and draining her magic. Just before dropping dead Alexanders tells Harper that now, with David gone, her powers, as well as Bee's and Ryan's will also fade.