The Corrupted Seren Stone is the largest piece of Seren which is located in the Hefin cathedral in the Hefin clan district in the north-west part of Prifddinas. Inside the cathedral reside Hefin monks. These monks sell cleansing crystals which can be used on the Corrupted Seren Stone for 9,800 prayer experience each, if uninterrupted. When the Voice of Seren is active in the Hefin Clan district, cleansing crystals will give an additional 20% prayer experience, for a total of 11,760 experience.