Helm’s Cathedral was, as the name indicates, once dedicated to the god Helm, Lord of Guardians. The Watcher. Many year ago, after Helm was lost to the world, the cathedral became a sanatorium and hospital, tending to the aged, the mad, and those too injured to recover. Even as a a facility for the unwell, it was built by followers of Helm, and is well-fortified; even a few dozen armsmen could hold it for some time.
When the Spellplague occurred, those afflicted by it – the spellscarred – found it a natural destination. Eventually, a young woman, the Prophet Rohini, made her way to the cathedral, and took charge of the treatment of its patients; she also sees visitors, frequently answering any questions asked with cryptic prophecies and omens. The cathedral is home to several hundred ill individuals, many of them spellscarred, and several dozen watchers, as well as a number of clergy – some who still worship Helm, even after his death, and some who followed the Prophet from the House of Knowledge as followers of Oghma.