Forinthry is an ancient name for the area now known as the Wilderness.
Forinthry is mentioned briefly by the Museum Curator in the Varrock Museum. He refers to several of the artefacts on display at the museum being related in one way or another to Forinthry. The Curator believes that Forinthry was once a great city, much like Varrock is today, and that it was destroyed during the God Wars.
Armies of Gielinor
Forinthry is also referenced in the RuneScape-based FunOrb game, Armies of Gielinor, in the briefing for the second mission of the single-player campaign, "Guthix Awakens". One of the Sages of Guthix, Ewean, says that, "[The] land is condemned, all the way from Forinthry to Sophanem." Little information could be gleaned from this, other than the implication that it was far from Sophanem.