All of the priestesses residing at the temple were slain and their trove was looted. This attack was orchestrated by Hybern, to gain one part of the Cauldron that was hidden there. The places where the other two parts were: Sangravah and Itica were also attacked.
Morrigan tells Rhysand the news of the priestesses' death. Feyre Archeron also hears this news as she was standing nearby. When Feyre returns to the Spring Court, she relays this information to Tamlin.
A while after, Rhysand visits Cesere to meet with the few surviving priestesses and discuss rebuilding the temple. He writes to Feyre while he is there.