Banefire is the fail-safe that destroys what's hidden under Demonreach island. It is first mentioned in Cold Days.
Banefire appears as a scarlet and blue light becoming a white destructive fire which would kill the creatures imprisoned under Demonreach rather than allowing them to escape.
In Cold Days, Demonreach shows the characteristics of Banefire to Harry Dresden, through Bob's projection. According to Bob, a directed burst of energy unraveling the containment spell Merlin would trigger the fire called the "failsafe".