Blood Boil Rune
When invoked deals up to 30 burn damage to nearby non-cultists and sets them on fire.
On activation the rune charges for six seconds, changing colour, glowing, and giving out a warning to all nearby mobs. After the charging period the rune burns any non-cultists in view and sets them on fire. After another short wait it does the same again with slightly higher damage. If the cultists channeling the rune move away or are stunned at any point, the rune is deleted. So it can be countered pretty easily with flashbangs.