Prifddinas is the northern area of the elven lands of Arandar. It's currently closed, but it is often speculated to be a future part of the Mourning's Ends series (it is, however, accessible in RuneScape 3). It's surrounded by walls made from Elven Crystal. It is said to be the first RuneScape city ever built before the great God Wars. The Elf Crystal walls defended it. No other information has been released.
Prifddinas was accessible during the first Old School Runescape birthday event.