A Sunstorm Truce was a long-standing practice on planet Obsidian, wherein all hostilities between warring factions are put aside at times of severe solar activity, and all share shelters from the heat and radiation peacefully. It was regarded as Obsidian's oldest and most sacrosanct law, and when Sered refused to honor it, it caused the Faithful to turn against him. (TOS novel: Vulcan's Forge)