Maintenance for the week of March 16:
• PC/Mac: No maintenance – March 16
• ESO Store and Account System for maintenance – March 18, 6:00AM EDT (10:00 UTC) - 4:00PM EDT (20:00 UTC)
The effect is bugged and broken and almost all of these Rune Priest weapons, ZOS has yet to comment on it or implement any fix - based on prior behavior, I very much doubt they will. Quality control has appalling lately.
That blot is essentially a broken misaligned rune.