The name given to the mansion Dora viewed in her first dream. She thought of it as “the home of a deranged hoarder,” but didn’t know where it was located.
In Chapter 37, the party learned that Iris Hill was the Lowls family estate in Thrushmoor. Observed from the lake, it appeared as a compound surrounded by an impenetrable 20-foot-tall hedge wall, clearly magical in origin.
In Chapter 39, the party learned from Cesadia Wrentz that Melisenn Kororo, the head housekeeper/butler for the Lowls family, had fired most of the staff of the mansion shortly before the beginning of the campaign, and that since the earthquake she had sealed the estate.
During the botched “Great Harvest” ritual in Chapter 51, Carcosa was briefly viewed as co-existing with Thrushmoor. In the Carcosa-version of Thrushmoor, Iris Hill was absent, replaced by a windowless tower with battlements.
During a psychometry reading in the same chapter, Dora got a view of the sitting room of the estate, and came to understand that most of the rooms and objects she envisioned while casting her spells were drawn from the location.
First Reference: Chapter 26
Other Notable References: Chapter 37, Chapter 39, Chapter 51